ah! todays svn version works in ff3... sort of.
any menu/context-menu option gives the "select public key" window and does nothing else...

the xpi install fine, and the "decrypt this message" link in gmail works. setting preferences through the addons list works.
the connection with gpg-agent works fine as well, great!

anything I can do to help testing? I'm not a firefox coding adept, but getting my hands dirty with code is no prob.
I'm on:
Mozilla/5.0 (X11; U; Linux x86_64; en-US; rv:1.9b3) Gecko/2008021416 Firefox/3.0b3