Restart Firefox is a Firefox add-on that doesn’t even require to be presented: it just restarts your browser to apply changes or complete extensions installations, just by clicking a button.
There’s not much to say about it, since it is the simplest extension I have ever seen.
In fact, it is made of only one button that you can place anywhere on your toolbar and, when you click it, the browser asks for confirmation and restarts, applying the changes you made.
Free to download and use.