Beta 12 is going to be the last Firefox 4 beta, so it will be the last TenFourFox beta too. There will be a couple extra 10.4Fx specific fixes, hopefully.
It's unclear when Fx4/Gecko 2 will get its own hg branch. Once it does, this will start the countdown to see if we jump to Firefox 5 with them on mozilla-central. Remember, it's not guaranteed that we will retain compatibility, and there is always the risk they could require a 10.5-only feature at any time, though a serious widget library incompatibility isn't likely to happen until they require 10.6.
Once Fx4 moves to a stable branch, changesets will be issued against that stable branch, and the unstable branch will be idle for an indeterminate period until Firefox 5 goes through a sufficient number of betas. More about that when it actually happens, and knowing Mozilla, I don't see this happening much before summertime.
By the way, Flash 10.2 came out, and as threatened it is Intel-only. It is unclear if there will be any more 10.1 fixes, and to even get 10.1 you have to find their archived Flash Player versions page. I'd advise you don't hold your breath over continued support. Flash Player 9 has been completely deprecated, so if you must use Flash, use Flash 10.1.