main/postmarketos-config-firefox/prefs.js: tweaks to make Firefox even more useable on mobile

Modifies a few more Firefox options to make it slightly more usable on mobile devices.

Original source: https://gitlab.com/mobian1/mobian-tweaks/-/blob/master/tweaks/firefox-esr/mobian.cfg

I've only added the ones I feel make the most noticeable difference, without changing setting that may have negative side-effects.