It seems that I have to build my own 64-bit Firefox for Linux, as Mozilla won't support it until Firefox 4.
Why is this? It looks to me as though it works fine, although without some of the speed improvements to the Javascript engine which the 32-bit version gets.
(Edit: Yes I could run the 32-bit version but I'm trying to keep my system clear of 32-bit cruft and libraries etc, and all the plug-ins worked fine in 3.0.11 64-bit unofficial builds.)