Start Firefox in Diagnose Firefox issues using Troubleshoot Mode to check if one of the extensions is causing the problem (switch to the DEFAULT theme: Firefox (Tools) > Add-ons > Appearance/Themes).

• Don't make any changes on the Safe mode start window.
• https://support.mozilla.com/kb/Safe+Mode

• https://support.mozilla.com/kb/Troubleshooting+extensions+and+themes

This can be a problem with the file(s) that store the extensions registry.

Delete the files extensions.* (extensions.sqlite, extensions.rdf, extensions.cache, extensions.ini) and compatibility.ini in the Firefox profile folder to reset the extensions registry.
New files will be created when required.

See "Corrupt extension files":

• http://kb.mozillazine.org/Unable_to_install_themes_or_extensions

If you see disabled, not compatible, extensions in "Tools > Add-ons > Extensions" then click the Tools button at the left side of the Search Bar (or click the "Find Updates" button in older Firefox versions) to do a compatibility check or see if there is an update.

Well, I played around in Mozilla and finally went into McAfee and removed Site Advisor 3.3.1. So far, the error message hasn't appeared in one of the annoying areas. Will let you know after further testing if the problem has been completely eliminated or not. Thanks.

Please click the Solved It button next to the answer that solved your Firefox support issue, it appears when you are logged in, so this thread gets marked as Solved to help other users who may have this same problem.

Thanks for telling me about this--I didn't see it and didn't know I should click it.