This note relates to Mozilla Firefox 3.5.7 running on Ubuntu Karmic (9.10).
The problem is that Firefox appears to be freezing for a quick moment, every 4 to 5 seconds. This is annoying when editing text and watching a video on YouTube.
I came across this helpful post: http://support.mozilla.com/en-US/forum/1/524464
My approach to the fix is to give up on session restore, altogether. I do not like it, anyway. The post above might help you with tweaking better values if you wish to retain this feature.
1. Open firefox to this page: about:config
2. In the filter box, type: browser.sessionstore
3. Double click on “browser.sessionstore.interval” and enter a larger value of your choice (I used 100000, but the next step might make this irrelevant)
4. Double click on “browser.sessionstore.resume_from_crash”, resetting the value (i.e. make it “false”)
This fixed the annoying behaviour. I hope it works for you.