To uninstall Firefox on Windows, follow these steps:

1. Exit Firefox. From the menu at the top of the Firefox window, select File and then select the Exit menu item.
2. From the Start Menu in the Windows taskbar, select Control Panel. The Control Panel opens.
   
   
   
3. Open the Windows program listing.
   • (Windows Vista) Click Uninstall a program.
   • (Windows 2000/XP) Double-click Add or Remove Programs.
4. From the list of currently installed programs, select Mozilla Firefox
   
   
   
5. To begin the uninstall:
   • (Windows Vista) Click Uninstall.
   • (Windows 2000/XP) Click Remove.
   
   
   If the Uninstall Wizard does not run, you can start it manually, by running C:\Program Files\Mozilla Firefox\uninstall\helper.exe.
6. In the Mozilla Firefox Uninstall Wizard that opens, click Next.
7. Click Uninstall.
   • If Firefox is still open, you must close Firefox to proceed with the uninstall.
   • If you want to remove your Firefox user data and settings, put a check mark in the box that says Remove my Firefox personal data. If you select this option, Firefox will not preserve your bookmarks, saved passwords, and other data if it is installed again.
   • To provide feedback on your Firefox experience to the development team, select Tell us what you thought of Firefox.
8. Click Finish.
9. To remove added plugins or other files and folders that may not be removed by the uninstall you must manually delete the Firefox installation folder, which is located here by default: C:\Program files\Mozilla Firefox

1. Open the Applications folder.
2. Drag the Firefox application to the Trash.

Your uninstall of Firefox is finished.

If you installed Firefox with the distro-based package manager, you should use the same way to uninstall it - see Installing Firefox on Linux for details.
If you downloaded and installed the binary package from the Firefox dowload page, simply remove the folder firefox in your home directory to uninstall Firefox.