First I've heard of this as I use both IE8 and Firefox regularly in both Vista Ultimate and Windows 7 Uktimate x64 and have had no crashes. It has happened with beta versions of browsers but shouldn't be happening with regular versions. The only connection McAfee would have with the actual Firefox browser itself would be through SiteAdvisor as it uses Internet Explorer for display and communication. Have you tried disabling SiteAdvisor if you have it installed?
Having all firewalls down may be necessary but it's certainly unsafe. I would try scanning for spyware using the free versions of these two tools. Update them before running and let them remove anything they find and reboot when & if asked to.
You may also want to contact Technical Support Chat to see if they have heard of similar cases. See the McAfee Support link at the top of this page
What information do you have in the FireFox crash log, that might help pin point the problem
Have you tried uninstalling firefox? you need to remove your profile as well which is stored in a different place
There was an issue with earlier version of Firefox crashing but that seems to have been fixed with version 3.5.6 is yours up to date?