Did you install it from Mozilla's Add-ons site, or from the developer's site?

from mozilla's, but at this point, I"m not concerned with the addon so much as I am with the fact that even though I upgraded to 4.0 everything is still saying 3.5.5?

Try running Firefox in Diagnose Firefox issues using Troubleshoot Mode. If it functions properly in that configuration, then one of your add-ons is the culprit.

It? assuming firefox; and addon still says I have 3.5.5 as well as the system details.

This is typically caused when an add-on changes the user agent string (identifies the version of Firefox). To rectify this you can reset your user agent string.

1. Type about:config into the location bar and press enter
2. Accept the warning message that appears, you will be taken to a list of preferences
3. Locate the preference general.useragent.override, right-click (or control-click) on it and select Reset, this will set the user agent string back to the default in Firefox 4

There are other user agent string settings that may have been modified, but these are no longer used in Firefox 4. You can reset them if you want, for details see https://support.mozilla.com/kb/Websites+or+add-ons+incorrectly+report+incompatible+browser

In addition to Tony's answer. Your user agent show that you have Firefox 3.5.5, but also shows that you have the Fast Browser Search toolbar (FBSMTWB) installed that can be considered as malware.

Mozilla/5.0 (Windows; U; Windows NT 5.1; en-US; rv:1.9.1.5) Gecko/20091102 Firefox/3.5.5 (.NET CLR 3.5.30729) FBSMTWB

You can see if you can find it here:

Control Panel > Add or Remove Programs. Uninstall "Fast Browser search protection" and "Fast Browser Search" (MyWebTatoo)

Some files that you can search for:

fast-browser-search.xml, fbstoolbar.jar, fbstoolbar.manifest

See these pages and threads about Fast Browser Search (FBSMTWB in the user agent, Help > About).

• http://help.fastbrowsersearch.com/
• http://www.tattoodle.com/uninstall.aspx
• http://www.pccybertek.com/2009/05/remove-fast-browser-search

Thanks so far for your help, learning a lot I must admit, and here is where we are now. It didn't say I need 4.0 this time so I feel like we're almost there ...

This is what I got:

"The addon from services.addons.mozilla.org could not be installed because it does not match the add-on Firefox expected" and 3.5.5 still shows up there.

and that FBS... thing is still there :(

Did you reset the user agent?

You can reset the pref general.useragent.override on the about:config page.

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.

I did that already as per the earlier suggestion, and I've since uninstalled firefox 4.0 completely, and then reinstalled it.

and just an FYI and it does NOT let me reset compatMode.firefox, or .locale they are shaded grey

I don't know which of the above suggestions helped solve the problem, but I also "uninstalled", and reinstalled the ones I really use, and so far everything seems to be okay.

Thanks for your help.