See:

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

This can also 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.

That did it. Did not find files extensions.cache; extensions.ini or compatibility.ini in the Firefox profile folder, but found and deleted the others. Also discovered Firefox 3 was still installed and removed it via control panel, add or remove programs. Noted, Firefox 5 is installed and currently running, but short cut is titled Firefox 4.0 Beta 12. Thanks.

You're welcome.

You can rename the Firefox shortcut.

Make sure that you are on the release update channel.

It is possible that you are still on the beta update channel if you ever installed a Firefox 4 beta version or a release candidate.
You can check that by looking at the pref app.update.channel on the about:config page.
If the value of that pref is beta then it is better to download and install the latest Firefox 4.0.x release.

To open the about:config page, type about:config in the location (address) bar and press the "Enter" key, just like you type the url of a website to open a website.
If you see a warning then you can confirm that you want to access that page.
You can use the Filter bar at to top of the about:config page to locate a pref more easily.

• http://www.mozilla.com/en-US/firefox/all.html