Does what it says it would. My only gripe is that it seems to merge everything into whichever window has the most tabs open rather than whichever window you actually clicked on the button in. But that is a minor annoyance.

When I click the button, it merge the current windows to another one. Illogical.

Thank you for creating this add on for Firefox! Have been a long time user from the Chrome version.

Working as expected!