Firefox: provide opinionated postmarketOS configuration

Let's pre-configure Firefox in postmarketOS, so it serves the user in the best possible way:

• don't be distracting (no reminders of "hey check out this feature", no mozilla announcements on the start page, no advertising)
• respect user's privacy (no telemetry, no tracking through advertising)
• be energy efficient (no advertising)

To achieve these goals, we need to:

• disable the gift icon (top right icon here)
• disable "firefox pocket"
• disable telemetry
• set a blank homepage
• set a privacy friendly search engine (duckduckgo comes to mind)
  • DDG had a major privacy screwup here, but at least they fixed it now...
• install a free software adblocker addon by default (ublock origin?)

Maybe put it into a postmarketos-firefox-config package, which we add as recommended package to UIs where we install firefox by default (currently Phosh). Then it would be possible to uninstall this postmarketOS specific configuration. (Note that recommended packages are only supported by postmarketos-images as of writing, support in pmbootstrap is planned: pmbootstrap#1933 (closed).)

EDIT: I just noticed that we have an user-agent in epiphany that is easy to fingerprint (!1402 (merged)). I imagine that the situation is better with stock Firefox, but we should verify it and, if necessary, spoof the user-agent as well.