Hi,
I have the automatic logout problem as well. I'm currently using a Mac with Firefox running Joomla! 1.0.11. The only way to alleviate the problem I found is with (I hesistate to say) a PC using Internet Explorer. You have to make sure you delete your cookies before you log in. It might be it's looking at an older cookie thus thinking it's time to logout. I also set the internet security setting to 'medium' and deleted the cache and history for good measure. I haven't tested which one is the magic bullet, but I know I don't have the problem anymore using this combo.
Let me know if you find anything for Firefox.
Good luck,
Dave