It was weird man, i was in the dev forums and there were previous requests for this feature. And they wouldnt put it on the config page because users might not find it there. So cant have it at all.
Yes i have it mapped to a thumb mouse button for other applications. And then im browsing firefox, i wanna switch to a tab on the left, the button bumps against the keyboard, tab gets closed.
Are you aware of ctl + shift + t to reopen a closed tab? You can hit it multiple times too. Totally fine if you prefer this solution, but this is a useful tip regardless!
you cant bind middle mouse to anything in firefox, or in any browser I looked at. and I bound this button to middle mouse myself, it's used a lot in Blender and this is more comfortable than the default.