diff --git a/src/index.js b/src/index.js
index 98f4981f5f..f13737ef8f 100644
--- a/src/index.js
+++ b/src/index.js
@@ -1,6 +1,7 @@
 /* This Source Code Form is subject to the terms of the Mozilla Public
  * License, v. 2.0. If a copy of the MPL was not distributed with this
  * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+// @flow
 
 import React from 'react';
 import { render } from 'react-dom';
@@ -12,11 +13,12 @@ import {
   addDataToWindowObject,
   logFriendlyPreamble,
 } from './utils/window-console';
+import { ensureExists } from './utils/flow';
 
 // Mock out Google Analytics for anything that's not production so that we have run-time
 // code coverage in development and testing.
 if (process.env.NODE_ENV === 'development') {
-  window.ga = (event, ...payload) => {
+  window.ga = (event: string, ...payload: mixed[]) => {
     const style = 'color: #FF6D00; font-weight: bold';
     console.log(`[analytics] %c"${event}"`, style, ...payload);
   };
@@ -39,7 +41,13 @@ window.geckoProfilerPromise = new Promise(function(resolve) {
 
 const store = createStore();
 
-render(<Root store={store} />, document.getElementById('root'));
+render(
+  <Root store={store} />,
+  ensureExists(
+    document.getElementById('root'),
+    'Unable to find the DOM element to attach the React element to.'
+  )
+);
 
 addDataToWindowObject(store.getState);
 if (process.env.NODE_ENV === 'production') {
diff --git a/src/profile-logic/process-profile.js b/src/profile-logic/process-profile.js
index 196efd7d2d..49cbb89d86 100644
--- a/src/profile-logic/process-profile.js
+++ b/src/profile-logic/process-profile.js
@@ -6,7 +6,6 @@
 import { getContainingLibrary } from './symbolication';
 import { UniqueStringArray } from '../utils/unique-string-array';
 import { resourceTypes, emptyExtensions } from './profile-data';
-import { provideHostSide } from '../utils/promise-worker';
 import { immutableUpdate } from '../utils/flow';
 import {
   CURRENT_VERSION,
@@ -688,7 +687,7 @@ function _processThread(
     tid: thread.tid,
     pid: thread.pid,
     libs,
-    pausedRanges,
+    pausedRanges: pausedRanges || [],
     frameTable,
     funcTable,
     resourceTable,
@@ -926,8 +925,3 @@ export class ProfileProcessor {
     });
   }
 }
-
-export const ProfileProcessorThreaded = provideHostSide(
-  'profile-processor-worker.js',
-  ['processProfile']
-);
diff --git a/src/profile-logic/profile-store.js b/src/profile-logic/profile-store.js
index 6269105eb1..5d1fe87a0a 100644
--- a/src/profile-logic/profile-store.js
+++ b/src/profile-logic/profile-store.js
@@ -1,11 +1,12 @@
 /* This Source Code Form is subject to the terms of the Mozilla Public
  * License, v. 2.0. If a copy of the MPL was not distributed with this
  * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+// @flow
 
 export function uploadBinaryProfileData(
-  data,
-  progressChangeCallback = undefined
-) {
+  data: string,
+  progressChangeCallback?: number => void
+): Promise<string> {
   return new Promise((resolve, reject) => {
     const xhr = new XMLHttpRequest();
 
diff --git a/src/profile-logic/range-filters.js b/src/profile-logic/range-filters.js
index 8c159bc655..011930354c 100644
--- a/src/profile-logic/range-filters.js
+++ b/src/profile-logic/range-filters.js
@@ -1,8 +1,11 @@
 /* This Source Code Form is subject to the terms of the Mozilla Public
  * License, v. 2.0. If a copy of the MPL was not distributed with this
  * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+// @flow
 
-export function parseRangeFilters(stringValue = '') {
+import type { StartEndRange } from '../types/units';
+
+export function parseRangeFilters(stringValue: string = ''): StartEndRange[] {
   if (!stringValue) {
     return [];
   }
@@ -11,11 +14,13 @@ export function parseRangeFilters(stringValue = '') {
     if (!m) {
       return { start: 0, end: 1000 };
     }
-    return { start: m[1] * 1000, end: m[2] * 1000 };
+    return { start: Number(m[1]) * 1000, end: Number(m[2]) * 1000 };
   });
 }
 
-export function stringifyRangeFilters(arrayValue = []) {
+export function stringifyRangeFilters(
+  arrayValue: StartEndRange[] = []
+): string {
   return arrayValue
     .map(({ start, end }) => {
       const startStr = (start / 1000).toFixed(4);
@@ -25,7 +30,7 @@ export function stringifyRangeFilters(arrayValue = []) {
     .join('~');
 }
 
-export function getFormattedTimeLength(length) {
+export function getFormattedTimeLength(length: number): string {
   if (length >= 10000) {
     return `${(length / 1000).toFixed(0)} sec`;
   }
@@ -35,7 +40,7 @@ export function getFormattedTimeLength(length) {
   return `${length.toFixed(0)} ms`;
 }
 
-export function getRangeFilterLabels(rangeFilters) {
+export function getRangeFilterLabels(rangeFilters: StartEndRange[]): string[] {
   const labels = rangeFilters.map(range =>
     getFormattedTimeLength(range.end - range.start)
   );
diff --git a/src/profile-logic/symbol-store-db-worker.js b/src/profile-logic/symbol-store-db-worker.js
deleted file mode 100644
index f2880f781b..0000000000
--- a/src/profile-logic/symbol-store-db-worker.js
+++ /dev/null
@@ -1,8 +0,0 @@
-/* This Source Code Form is subject to the terms of the Mozilla Public
- * License, v. 2.0. If a copy of the MPL was not distributed with this
- * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
-
-import SymbolStoreDB from './symbol-store-db';
-import { provideWorkerSide } from '../utils/promise-worker';
-
-provideWorkerSide(self, SymbolStoreDB);
diff --git a/src/test/fixtures/mocks/file-mock.js b/src/test/fixtures/mocks/file-mock.js
index 602eb23ee2..acbcda64f1 100644
--- a/src/test/fixtures/mocks/file-mock.js
+++ b/src/test/fixtures/mocks/file-mock.js
@@ -1 +1,6 @@
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+// @flow
 export default 'test-file-stub';
diff --git a/src/test/fixtures/mocks/image.js b/src/test/fixtures/mocks/image.js
index 95465f468d..c133462c51 100644
--- a/src/test/fixtures/mocks/image.js
+++ b/src/test/fixtures/mocks/image.js
@@ -2,6 +2,7 @@
  * License, v. 2.0. If a copy of the MPL was not distributed with this
  * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
 
+// @flow
 export function createImageMock() {
   function Image() {
     instances.push(this);
diff --git a/src/test/fixtures/mocks/style-mock.js b/src/test/fixtures/mocks/style-mock.js
index ff8b4c5632..fc45854879 100644
--- a/src/test/fixtures/mocks/style-mock.js
+++ b/src/test/fixtures/mocks/style-mock.js
@@ -1 +1,6 @@
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+// @flow
 export default {};
diff --git a/src/types/globals/Window.js b/src/types/globals/Window.js
index 28f526569e..3b222d560f 100644
--- a/src/types/globals/Window.js
+++ b/src/types/globals/Window.js
@@ -20,6 +20,7 @@ declare class Window extends EventTarget {
   ga?: GoogleAnalytics;
   // perf.html and Gecko Profiler Addon
   geckoProfilerPromise: Promise<GeckoProfiler>;
+  connectToGeckoProfiler: GeckoProfiler => void;
   geckoProfilerAddonInstalled?: () => void;
   isGeckoProfilerAddonInstalled?: boolean;
   InstallTrigger?: {
diff --git a/src/profile-logic/profile-processor-worker.js b/src/types/libdef/npm-custom/photon-colors_vx.x.x.js
similarity index 56%
rename from src/profile-logic/profile-processor-worker.js
rename to src/types/libdef/npm-custom/photon-colors_vx.x.x.js
index 60d23d644c..98d1b440f6 100644
--- a/src/profile-logic/profile-processor-worker.js
+++ b/src/types/libdef/npm-custom/photon-colors_vx.x.x.js
@@ -1,8 +1,12 @@
 /* This Source Code Form is subject to the terms of the Mozilla Public
  * License, v. 2.0. If a copy of the MPL was not distributed with this
  * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+// @flow
 
-import { ProfileProcessor } from './process-profile';
-import { provideWorkerSide } from '../utils/promise-worker';
+declare module 'photon-colors' {
+  declare module.exports: any;
+}
 
-provideWorkerSide(self, ProfileProcessor);
+declare module 'photon-colors/photon-colors.css' {
+  declare module.exports: any;
+}
diff --git a/src/types/libdef/npm/photon-colors_vx.x.x.js b/src/types/libdef/npm/photon-colors_vx.x.x.js
deleted file mode 100644
index dff11e1034..0000000000
--- a/src/types/libdef/npm/photon-colors_vx.x.x.js
+++ /dev/null
@@ -1,39 +0,0 @@
-// flow-typed signature: 2677a4eb39233911257298be43e809dc
-// flow-typed version: <<STUB>>/photon-colors_v2.0.1/flow_v0.66.0
-
-/**
- * This is an autogenerated libdef stub for:
- *
- *   'photon-colors'
- *
- * Fill this stub out by replacing all the `any` types.
- *
- * Once filled out, we encourage you to share your work with the
- * community by sending a pull request to:
- * https://github.com/flowtype/flow-typed
- */
-
-declare module 'photon-colors' {
-  declare module.exports: any;
-}
-
-/**
- * We include stubs for each file inside this npm package in case you need to
- * require those files directly. Feel free to delete any files that aren't
- * needed.
- */
-declare module 'photon-colors/build' {
-  declare module.exports: any;
-}
-
-declare module 'photon-colors/photon-colors' {
-  declare module.exports: any;
-}
-
-// Filename aliases
-declare module 'photon-colors/build.js' {
-  declare module.exports: $Exports<'photon-colors/build'>;
-}
-declare module 'photon-colors/photon-colors.js' {
-  declare module.exports: $Exports<'photon-colors/photon-colors'>;
-}
diff --git a/src/types/reducers.js b/src/types/reducers.js
index 8ee154db22..97c2c3f69f 100644
--- a/src/types/reducers.js
+++ b/src/types/reducers.js
@@ -117,11 +117,6 @@ export type ZippedProfilesState = {
   expandedZipFileIndexes: Array<IndexIntoZipFileTable | null>,
 };
 
-export type RangeFilterState = {
-  start: number,
-  end: number,
-};
-
 export type UrlState = {|
   dataSource: DataSource,
   hash: string,
@@ -131,7 +126,7 @@ export type UrlState = {|
   profileSpecific: {|
     implementation: ImplementationFilter,
     invertCallstack: boolean,
-    rangeFilters: RangeFilterState[],
+    rangeFilters: StartEndRange[],
     selectedThread: ThreadIndex | null,
     callTreeSearchString: string,
     threadOrder: ThreadIndex[],