OBVIOUSLY I RESTARTED, AROUND 10 TIMES BOTH FIREFOX AND WHOLE PC, AS I SAID IT SIMPLY DOESN'T WORK Rated 1 out of 5 stars
It works only the first restart, then it doesn't work anymore and needs to be reinstalled everytime.This user has a previous review of this add-on.
This is very simple extension, just a few lines of code. If it doesn't work, may be it conflicts with the other extension that you installed.
Or you can try the first version https://addons.mozilla.org/en-US/firefox/addon/double-click-to-reload-tab/versions/?page=1#version-1.0.0
If it still doesn't work, I can't help you anymore. Sorry!