5b2 is being delayed a bit, for two reasons: 1) I want to get the complete AltiVec-enabled YCbCr-to-RGB conversion suite into it for testing (one part of four is in 5b1; I've finished a second part and am working on parts 3 and 4) and 2) bug 662450, which yours truly reported and is a Mozilla bug, but disproportionately affects us as we are unaccelerated. Mozilla is currently treating it as a blocker, so there should be some resolution to it soon.
As soon as the two issues are completed, I will run off new builds, which also fix issue 65 on Leopard. We will probably be able to get 5 final out on time with Firefox 5, and I plan to release 4.0.3 at the same time as the final release in the 10.4Fx 4 series. I have not seen any new landings on the Mozilla beta tree that would obviously break plugins, so assuming they still work, they will still stay. However, a big change landed on the tree for Firefox 7 that could potentially break them, so I still plan to pref them off in TenFourFox 6.