I ASSUME it's only crashing with high volume streaming on flash?
Do you have multiple web-pages open at a time?
Check to make sure your net isn't cutting out sometimes. Are you using a wi-fi signal or wired?

Do you use any anti-virus scanners like AVG which has a web link scanner? They might be interferring while doing their scan.

Update your graphic card drivers to the latest version (if not already) as it might be an issue with 'graphic' hardware acceleration on flash.


Also note:
Mozilla is urging all users to update to FIREFOX 3.6.6 in order to fix performance slowdowns and problems with the most recent Adobe Flash plugin version 10.1.63.54, installed as part of Flash Player 10.1. Wrote in a blog post that Firefox 3.6.6 brought along enhancements to the crash protection feature, first introduced in Firefox 3.6.4.

Crash protection, also called out-of-process plugin support, basically allows you to reload the web page when a plugin like Flash or Silverlight becomes unresponsive rather than bring down the entire browser. Firefox 3.6.6 simply extends the amount of time it’ll wait before terminating unresponsive plugins:

Quote

Following the release of Firefox 3.6.4 we heard from some users, mainly those using older computers, that they sometimes expect longer periods of non-responsiveness from plugins, especially with games. For these users the default timeout of 10 SECONDS was too short. To address this, we increased the amount of time Firefox waits for a plugin to respond before terminating it from 10 to 45 seconds.