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/puppeteer-firefox/misc/puppeteer.cfg

@@ -137,6 +137,9 @@ pref(
   "http://%(server)s/dummy/discoveryURL",
 );
 
+pref("extensions.screenshots.disabled", true);
+pref("extensions.screenshots.upload-disabled", true);
+
 // Allow the application to have focus even it runs in the background
 pref("focusmanager.testmode", true);