A few days ago Mozilla released Firefox 3.6.4 with Crash protection against plugins, however, it looks like they had to fix a few bugs with the release and have had to release Firefox 3.6.6.
As part of Mozilla’s ongoing stability and security update process, Firefox 3.6.6 is now available as a free download for Windows, Mac, and Linux from http://www.firefox.com. This release makes a small change to the crash protection feature introduced in Firefox 3.6.4, increasing the amount of time that Firefox will allow a plugin to remain frozen before terminating it. For more information, see the Mozilla Blog announcement about the release.
You can always update the browser by going to help and using the Check for Updates option.