Mozilla has started yet another feature called Firefox Marketplace that offers you thousands of apps of various kinds. These apps do not need depend on the Firefox web browser and run directly using the operating system API just like any other application. This can confuse many people as they think the apps would appear in Firefox web browser after being installed, but they do not show anywhere in the Firefox browser. Instead a shortcut to these apps is created on the Windows desktop through which you can launch these apps.
I installed the SoundCloud app from Firefox Marketplace using Firefox running in Windows and it has created a shortcut to the app on the Windows desktop. The apps, their settings and cache etc., is all stored in the current user profile folder.
Since these apps are installed just like any other ordinary programs, they can easily be installed using the Programs & Features control section. All you have to do is open Control Panel, click on Uninstall a program under the Programs section. Then find the apps installed through Firefox Marketplace, right-click on them and select Uninstall from the menu.
The app uninstaller does not clean everything from your hard disk related to these apps. If you want to get rid of all the remnants left behind by these apps, then you have to delete them manually. In order to do this, open Windows File Explorer, type %AppData% in the location bar and press Enter. This will open your app data folder where you find the sub-folders belonging to the apps installed through the Firefox Markerplace. You can just go ahead and delete the sub-folders belonging to the apps you want to remove.
Conclusion: Firefox Marketplace has many nice apps to offer, but removing these apps is not really straightforward. You have to take multiple steps to remove these apps from your Windows system completely.