main/mobile-config-firefox: upgrade to 2.0.0

Full changelog: https://gitlab.com/postmarketOS/mobile-config-firefox/-/tags/2.0.0

Most notably:

• support convergence
  • the urlbar and all other tweaks are only applied when the firefox window is displayed on a small screen
  • compare the urlbar in the photo below from phone and external monitor (both windows belong to the same FF instance)
• urlbar: don't do extra extension to left and right

Example show case:

• open a remark based slideshow like this one
• press "c" to clone it into a new window. Both windows are connected now, if you go to the next slide in one window, it goes to the next slide in the other window.
• move the new window to your external monitor
• swipe left and right on your phone, and use it to control the presentation on the monitor! :D