I have this problem too with the 64-bit version of Hardy.
It doesn't seem to be specifically related to Firefox 3. It also happens in Epiphany but not Konqueror.
It still happens if I run Firefox 2 on a remote machine, running gutsy, and use X11 forwarding over ssh.
It doesn't happen if I run firefox3 on hardy 64-bit with X11 forwarding to display it on OS X.
I have this problem too with the 64-bit version of Hardy.
It doesn't seem to be specifically related to Firefox 3. It also happens in Epiphany but not Konqueror.
It still happens if I run Firefox 2 on a remote machine, running gutsy, and use X11 forwarding over ssh.
It doesn't happen if I run firefox3 on hardy 64-bit with X11 forwarding to display it on OS X.