Try the steps at How to reset the default user agent on Firefox, your user agent is corrupted

Hi, what's going on is that you have a "stuck" user agent. The user agent is what your browser tells websites so the sites can identify things like version and operating system.

At some point around Firefox 3.6.18 you installed the Fast Browser Search toolbar (likely through some third party program, and possibly not intentionally). It changed a setting in Firefox and "locked" the user agent. Even though you're updating the browser, the user agent isn't updating.

There are two ways to get it fixed:

You can either reset Firefox settings (easier) or change the settings manually:

The Reset Firefox feature can fix many issues by restoring Firefox to its factory default state while saving your essential information. Note: This will cause you to lose any Extensions, Open websites, and some Preferences.

To Reset Firefox do the following:

1. Go to Firefox > Help > Troubleshooting Information.
2. Click the "Reset Firefox" button.
3. Firefox will close and reset. After Firefox is done, it will show a window with the information that is imported. Click Finish.
4. Firefox will open with all factory defaults applied.

Further information can be found in the Refresh Firefox - reset add-ons and settings article.

Or, you can just change the user agent settings manually:

1. In the Location bar, type about:config and press Enter. The about:config "This might void your warranty!" warning page may appear.
2. Click I'll be careful, I promise!, to continue to the about:config page.
3. In the search box, type "general.useragent" (without quotes)
4. Right click on each bolded item that shows up and select Reset from the menu.

Hello pamclements, see also : http://help.fastbrowsersearch.com/

for me try to reset the useragent, as Tylerdowner said in his reply AND Cww said as second solution, it is easy you can do it.

thank you