Perfect.

Works great and is exactly what I needed.

This is awesomely slim and works fast – especially if you assign it some keyboard shorcuts!

This works perfectly and is such a game changer. I'm the kind of person who has a million tabs open at once so it's great to be able to combine similar ones if need be. I might sound a bit over dramatic fangirling over a browser extension but this thing is seriously great.

Exactly what I was looking for - simple and does the job perfectly. This is the only add-on I found that can merge two windows - the others force to merge all.

https://github.com/Zegnat/browserext-window-merger/issues/4#issuecomment-561804594 for use with Firefox Quantum.

Still, five stars because this extension is outstandingly good with Waterfox Classic.