Firefox has what i personally think is an incredibly annoying "feature" called 'tab detach' whereby you can drag a tab downwards in order to open it in a new window. The problem i have with this behavior is that i often trigger it accidentally when i'm clicking or switching tabs in a hurry.
If this feature makes you want to torture kittens, open
about:config in your address bar, search for
browser. and set the bloody thing to '
false'. Done. No extension needed. Better yet, add it to your custom user.js (or user-overrides.js if you're using one). I just learned of this new preference today, however it's been available since Firefox v74.
If you follow my Firefox Extensions page, you may have installed the Disable Tab Detach add-on. You can now dump that.
Thanks Mozilla for making us wait YEARS for this and double thanks for never adding this preference in the first place.