Adding this to my person WIKI of information…
In Firefox 126.96.36.199 on Linux (Ubuntu), the default behavior of the middle mouse button is to paste the contents of the clipboard as a url. No good that. Everyone knows that the middle mouse button is for closing tabs, right? (bet you Mac users are jealous now eh?).
To change it (remember, this is for Linux only, as far as I can tell), type about:config in the address bar and double click the following entries:
Double click those will change their value to false and make the middle mouse button work as it should.