Replace firefoxheadless.sh with soft kill

[BaerMitUmlaut]

• Fixes Kill the headless Firefox process by PID, not by grepping ps #61 (removed firefoxheadless.sh, save PID)
• Fixes firefoxheadless.sh kill fails on Mac #191
• Kills Firefox softly in tests instead of killing the process

I'm still having a little trouble with running my build which might be related to using WSL (I can't run master either)... Will test again later on Linux.

[BaerMitUmlaut]

It seems like previously the marionette was already gone and thus the quit command could not be sent, so I tried killing by PID. I also noticed that the firefoxheadless.sh is still needed because web-ext doesn't support a headless parameter.
One problem now is that the process is getting killed twice because of the deferred process kill. Maybe I can find a cleaner way.

[tombh]

Sorry for the delay. I've just been away from the codebase so long, I can't read this without running the code myself. So the only problem with it is for dev/test environments? Because of web-ext's reliance on firefoxheadless.sh? Maybe that's why the Travis build failed?

I wish I wasn't so long away from the code, then I could give you better advice. But thank you so much for contributing.