chore(juggler): Roll Firefox to 120450a2 (#3842)

This patch:
- rebaselines C++ patchset atop of [Global Firefox Reformat](https://bugzilla.mozilla.org/show_bug.cgi?id=1511181)
- rolls firefox to [120450a2](120450a2c5)
- splits out preference installation step from Puppeteer-Firefox's
  `install.js` into a separate
  `puppeteer-firefox/misc/install-preferences.js`. This script is
  re-used to install preferences when run with a custom executable path.
- fixes issue with ScrollbarManager that was re-injecting the same
  stylesheet multiple times

experimental/juggler/README.md

@@ -25,10 +25,18 @@ SOURCE=$PWD bash scripts/fetch_firefox.sh
 ```bash
 cd firefox
 git am ../patches/*
-ln -s $PWD/../src $PWD/testing/juggler
 ```
 
-4. Bootstrap host environment for Firefox build and compile firefox locally
+4. Add Juggler to Firefox. NOTE: On Linux, symlinks work. On OSX, files have to be copied.
+
+```bash
+# LINUX:
+ln -s $PWD/../src $PWD/testing/juggler
+# OSX:
+cp -r $PWD/../src $PWD/testing/juggler
+```
+
+5. Bootstrap host environment for Firefox build and compile firefox locally
 
 ```bash
 # OPTIONAL - bootstrap host environment.