[Update: Sorry for the confusion, but the Mac version of Firefox does not have the feature that protects against crashes in plug-ins. -Adam]
Mozilla's Firefox 3.6.6 [1] is a quick update to the just-released 3.6.4, which addressed a handful of security vulnerabilities and improved stability when plug-ins crash. Most notably, if the Flash, QuickTime, or Silverlight plug-ins crash, they won't take down Firefox itself. (Just reload the page to restart the plug-in.) The security vulnerabilities could result in arbitrary code execution, as usual, so it's nice to seem them fixed. Release notes [2] are available. Firefox 3.6.6 merely increases the amount of time a plug-in can be non-responsive before it's terminated. (Free, 19 MB)
[1]: http://www.mozilla.com/
[2]: http://www.mozilla.com/en-US/firefox/3.6.6/releasenotes/