Why does clicking on tabs in Firefox sometimes fail until I hit cmd-Q, and then why does that not quit but instead make clicking on the tabs work, and how do I stop that behavior?

I'm running Firefox 3.5.5 on Mac OS X 10.5 and this has been happening for a while (with previous versions of FF as well). I can't forcibly reproduce the behavior, but every now and then (few days?) I just can't click on tabs or on the x's to close the tabs. I can still ctrl-tab between tabs, though. But if I press cmd-Q, instead of quitting, Firefox seems to seize for a second and then I can click on tabs and click to close them just fine. No clue why this is happening or how to stop it. And I do have tons of extensions installed, so it's plausible one of them is the problem..... Thanks.