Are you blocking plugin_container.exe with your firewall, or did you kill the process? For details about how it functions, see: http://support.mozilla.com/en-US/kb/What%20is%20plugin-container
If Firefox is crashing, type: about:crashes in the location bar where you normally type in a web address and hit Enter. The random characters which appear on the following page are links. Find one which coincides with the time of a crash on youtube or some other video site and you might find some clues as to the cause.
Xircal, thanks for the info. Think I've finally figured it out though. Make a long story short, the ZA toolbar (either community or security, I uninstalled both) is incompatible with FF when flashplayer is installed and pulling up a youtube page. Once I uninstalled FF and Flash Player and Zone Alarm (without toolbars) worked like a charm.
thanks for your help. Just the support was helpful
Glad to hear you're up and running again.
Here's a list of add-ons which are problematical in Firefox just in case you come across any which are bundled with some other app: http://kb.mozillazine.org/Problematic_extensions