If you have an issue choosing the application to open Telegram links, you can open this link manually: https://telegram.me/share/url?url="test"
And check the check button to always open telegram links with telegram desktop. This is a Firefox setting, not a problem with the extension.

Another way of doing this: you can go to the Firefox settings, scroll down to the "Applications" part and make sure that for "tg" content type, "Action" is selected as "Use Telegram Desktop (default)