Normally extensions will add items to the 3 dot menu in the top right. If there is something the user needs to interact with. Adding buttons to the toolbar is not possible.

The other spot to check is the installed addons list. It can be reached by tapping the 3 dot button then tapping on tools and tap on addons. It can also be reached by typing about:addons in the address bar the the return/enter/go button on the keyboard. Tap on the addon in the addons list page and there may be additional settings for the extension shown.