Update Firefox desktop pages post GitHub migration

[bcolsson]

No description provided.

[eemeli]

It might be best not to mention gecko-dev at all; it's effectively an implementation detail that the mozilla-central contents are read via that git repo.

[flodolo]

It might be best not to mention gecko-dev at all; it's effectively an implementation detail that the mozilla-central contents are read via that git repo.

I think it needs to be called out, because it has some implications (delay, automation getting stuck). Most important, it's still very unclear when the move to git will happen.