FireGPG  works for me under Firefox 3.0 :) (both beta and nightly)
the only you need to do is modify install.rdf

(em:maxVersion="3.0.*")

There is also small problem with https requirement..
(em:updateURL="https://firegpg.tuxfamily.org/stable/update.rdf")
.. but it's not a problem of extension itself. Maybe it will be easier to put FireGPG on https://addons.mozilla.org/ ?