I've been preparing Firefox 4 since quite some time in mozilla:beta (and previously :alpha) and I run it everywhere meanwhile. The initial Firefox roadmap said it would have been ready by october this year but was slipped to something "early 2011" https://wiki.mozilla.org/Firefox/4/Beta which would still fit into the openSUSE 11.4 release.
There are still some issues and quite some not updated extensions but to get proper testing I think it makes sense to go for Firefox 4 in Factory soon. My plan is to do the update before Milestone 5 (actually right after milestone 4 to get a feeling before M5 gets done).
I do _not_ plan to have an easy fallback to Firefox 3.x in Factory but it will be available in the mozilla repository as always. The reasons are: 1. it's extra work to have a package available for parallel installation 2. the much more important one: There is always a risk for user profiles when doing major downgrades.
Please tell me if there are strong arguments not to do it this way.