Your browser is reporting to websites that you're using Firefox 3.5.5 for some reason. Do you have any Add-ons that might alter your User Agent or Headers? The easiest way to find out would be to check if the problem persists in Safe Mode (Help -> Restart with Add-ons Disabled), and then visit about:support (Help -> Troubleshooting Information) to check that your User Agent looks to be reporting the same as your version.

Your UserAgent string in Firefox is messed up by another program that you installed. http://en.wikipedia.org/wiki/User_Agent

type about:config in the URL bar and hit Enter
If you see the warning, you can confirm that you want to access that page.
Filter = general.useragent.extra.firefox
Right-click that preference and select Reset
Then restart Firefox

Also check other general.useragent prefs like general.useragent.override

See:

• https://support.mozilla.com/kb/Websites+or+add-ons+incorrectly+report+incompatible+browser
• http://kb.mozillazine.org/Resetting_your_useragent_string_to_its_compiled-in_default

Problem solved! Thanks for all the info, it all helped.

The problem was the "configuration mania" add-on. I don't even remember installing it!

Thanks again for all the advice!! You guys rock!!