tl;dr 4.0.x branching

In case John's branching blog post was tl;dr:

Mozilla branch	Mobile branch	Firefox desktop version	Firefox mobile (Fennec) version
mozilla-central	mobile-browser [1]	Firefox Next	Fennec Next
mozilla-2.0	N/A	Firefox 4.0[.x]	N/A
mozilla-2.1	mobile-2.0	N/A	Fennec 4.0[.x]

[1]: We're still debating whether we're going to merge mobile-browser in.

And for l10n:

Firefox desktop version	Firefox mobile (Fennec) version	L10n repo directory
Firefox Next	Fennec Next	l10n-central
Firefox 4.0[.x]	Fennec 4.0[.x]	l10n-mozilla-2.0 [2]

[2]: We're using the same set of l10n-mozilla-2.0 repos for both Firefox 4.0.x and Fennec 4.0.x.