Go to tools. There you see the add-on you install

Enable the addon toolbar then you can see most of the installed addons...

Both of those are Desktop Firefox answers.

On Firefox for Android many addons add to the application menu. This can be access by pressing your phone's hardware menu button. If your phone does not have a menu button then Firefox displays a 3 dot button to the right of the address bar.

If you still can't find the extension settings then please mention which extension you are trying to use.