Make sure your security software is up to date, and scan for viruses and other malware.
Maybe one of your add-ons is corrupt. You can either disable them all then test each one, or try Firefox in its own Safe Mode, (goto Start > Run, type in firefox -safe-mode and click OK). I assume you are using Windows XP?
The difficulty with that option is, occasionally means intermittent, so trying to catch this when that happens is going to be a problem.
Possibly your Firefox profile is corrupted. Have a look at John Wilkinson's [urlhttp://forums.cnet.com/5208-6620_102-0.html?threadID=247371&tag=forums06;forum-threads]Sticky note, (2nd post), for guidance how to recover from that. You may want to back up your bookmarks.html file first.
Check for patterns. Does this happen when a) a set number of tabs are open, b) a particular web site is accessed, c) particular web sites in the other tabs are open, or any combination of those.
Occasionally when using the latest version of firefox ( I cannot remember if this happened prior to the latest update) the entire page goes black in all tabs. I cannot do anything but close firefox and restart it. Does this sound like a virus or possibly just a bug within the latest version? Anyone else experience this? Any help or suggestions are appreciated. I dont do crazy downloads or go to suspicious web sites either, at least not on purpose.