Certain Firefox problems can be solved by performing a Clean reinstall. This means you remove Firefox program files and then reinstall Firefox. Please follow these steps:

Note: You might want to print these steps or view them in another browser.

1. Download the latest Desktop version of Firefox from http://www.mozilla.org and save the setup file to your computer.
2. After the download finishes, close all Firefox windows (click Exit from the Firefox or File menu).
3. Delete the Firefox installation folder, which is located in one of these locations, by default:
   • Windows:
     • C:\Program Files\Mozilla Firefox
     • C:\Program Files (x86)\Mozilla Firefox
   • Mac: Delete Firefox from the Applications folder.
   • Linux: If you installed Firefox with the distro-based package manager, you should use the same way to uninstall it - see Install Firefox on Linux. If you downloaded and installed the binary package from the Firefox download page, simply remove the folder firefox in your home directory.
4. Now, go ahead and reinstall Firefox:
   1. Double-click the downloaded installation file and go through the steps of the installation wizard.
   2. Once the wizard is finished, choose to directly open Firefox after clicking the Finish button.

Please report back to see if this helped you!

Where is Firefox stating you have Windows XP?

Also there are no separate Firefox 20.0 versions for different Windows versions. The Firefox 20.0.1 from mozilla.org is for Windows XP SP2 and newer including Windows 7 and 8. http://www.mozilla.org/en-US/firefox/20.0/system-requirements/

Check that you do not run Firefox in compatibility mode or with a reduced screen resolution.
Make sure that you do not run Firefox in compatibility mode.
You can open the Properties of the Firefox desktop shortcut via the right-click context menu and check the "Compatibility" tab.
Make sure that all items are deselected in the "Compatibility" tab of the Properties window.