Hold firefox updates

Because the upgrades are very slow, of no use to this project, and
lately have begun failing quite often.

.github/workflows/build.yaml

@@ -36,7 +36,7 @@ jobs:
     steps:
       - name: Upgrade OS
         run: |
-          sudo apt-mark hold grub-efi-amd64-signed
+          sudo apt-mark hold firefox grub-efi-amd64-signed
           sudo apt update && sudo apt upgrade
           sudo apt install lcov libfuse2 llvm
       - uses: actions/setup-python@v5
@@ -569,7 +569,7 @@ jobs:
       - uses: actions/checkout@v4
       - name: Install lcov
         run: |
-          sudo apt-mark hold grub-efi-amd64-signed
+          sudo apt-mark hold firefox grub-efi-amd64-signed
           sudo apt update
           sudo apt upgrade
           sudo apt install lcov