Firefox suddenly opens tabs in new window

[GeorgiaMutt]

I'm using Firefox 89.0 on Linux Mint 21 XFCE version. When I open a page in new tab, sometimes after a few seconds, it will suddenly close the new tab and open the page in a new window - a whole new iteration of Firefox. This has been going on for as long as I can remember, so it affects past versions of both Linux and Firefox. This is very annoying! Has anyone else encountered this quirk? Does anyone know what's causing it?

[RIH]

Does anyone know what's causing it?

It sounds like a browser re-direct.
What pages do the newly spawned tabs open to?
If it is to advertise items (particularly 'dubious' products/services) then it is almost certainly that.

If that is so then you have two choices.
1. Track down & remove the re-direct - probably from somewhere in the about:config tab. That can be time consuming in order to find the cause, although a 'google' search on 'Removing Firefox browser re-directs' should give you some valid clues.
2. Remove Firefox completely from your equipment - that will include some mozilla folders that just removing the Firefox app won't remove automatically. They can be found using Search from / in Nemo*. Then re-install Firefox, probably as a brand new user just to make sure.

Edit:
* sorry, Thunar in your case..

[all41]

I hate this--For me this was caused by inadvertent middle-clicks during scroll as this mouse is super sensitive.

In the FF url window enter:
about:config
read the warning and accept,
type 'middle' in the search field
There are several controllable operations--I have set all those to 'false'
good to go now

[GeorgiaMutt]

Thanks All41, I'm going to try that!

[GeorgiaMutt]

Thanks RIH, I'll look into that. The new windows are just opening the last tab I opened; they just break out into a separate instance of Firefox. I suspect this was meant to be a "feature" that somehow ran amok.