In order to uninstall a possibly unwanted extension, please do the following:

1. From the Firefox window click the Firefox button at the top left and select Add-ons, or, if the Firefox button is not shown, click the Tools menu and click Add-ons.
2. Once the Add-on Manager has opened in a new tab, click the Extensions button on the left side of the window.
3. You should now see a list of your installed extensions on the right side together with buttons on the right side of each extension.
4. To remove an extension from Firefox, simply click the Remove button. You should see a message that informs you about the successful removal of the add-on.
5. Note that some add-ons require a Firefox restart to be removed completely. To perform a Firefox restart after the add-on removal, click the Restart now link in the message.

You can find further information about uninstalling extensions in the following articles:

Disable or remove Add-ons

Remove a toolbar that has taken over your Firefox search or home page

Extensions that you install yourself should always have a Remove (uninstall) button.

Extensions that do not have a "Remove" button are installed by other software and are not under control of the Firefox extension manager.
Such globally installed extensions are usually found via a registry scan or are installed in a location that Firefox scans for installed extensions.

• https://developer.mozilla.org/en/Adding_Extensions_using_the_Windows_Registry
  

Extensions installed this way need to be removed via the settings (options/preferences) of the program that has added this extension or you need to uninstall this program via "Control Panel > Programs".
In Firefox you an only disable such an extension.