I’ve just upgraded to Firefox 3 Beta 4.
The spreadsheets component of Google Docs appears to work now. And although this suggests improvements in AJAX support, a known issue with GMail contacts remains:

GMail (new version) conversation labels appear on their own row in the message list, and names don’t show in the contacts manager (bug 415252)

This GMail bug remains a showstopper for me.
Of course, it’s important to remember that

Firefox 3 Beta 4 is a developer preview release of Mozilla’s next generation Firefox browser and is being made available for testing purposes only.