Firefox Keyboard problem?

Greetings. I'm using Firefox 1.0.7 on Mac OS 10.4.2. After a few
minutes of running Firefox, my keyboard becomes unresponsive, but just
within firefox. Command functions still work. For example, I can type
something in another application, copy it, and then switch to Firefox
and paste. This works both in forms and in the URL address field.
Quitting and restarting Firefox clears the problem, for a while anyway.

I am not using any Firefox extensions. 

Any ideas?