For the moment addons means UserJS, meaning for instance sites like https://openuserjs.org/ and https://greasyfork.org/.
However, as to your question itself, you don't need any addons for that. There are two main methods available.
In both cases, what you need is a bookmarklet that performs the action you want, namely something like this (via):
For a bookmarklet, you add a bookmark, put that in the address, and add a convenient keyword (like "mail") or whatever.
For a custom button, you right click on e.g. the back/forward buttons → Navigation Bar → Configure. Then you add one with the "Open URL" action.