Today, I managed to make time and install the Firefox 4 beta 3. Naturally, one of the first things I did was to check if Selenium IDE would work after bumping up the maxversion in the install.rdf file. It didn’t! 
The good news is that I managed to get it working after some trivial changes. I did not do an exhaustive test, but recording scripts and executing scripts, saving test case, saving test suite with a single test case, loading test case from recent menu, exporting test case in C#, Java, Perl, adding commands from right click menu, log messages and levels in log pane, reference pane and command auto-complete seemed to work fine. I will take it through more rigorous paces next week. All my Selenium IDE Plug-ins also seemed to work without requiring any changes. Hurray!!
Update: I have submitted a patch as issue 761 (http://code.google.com/p/selenium/issues/detail?id=761). Those wanting to try it out earlier can get the patch directly from there and bump up the maxversion in install.rdf to get the Selenium IDE to work in Firefox 4.