As someone who has gone out of their way to build Fenix for the literal sole purpose of implementing this functionality, I'd like to say that this feature is to *die* for. It saves so much time to not have to open the tab center or long-press that button each and every time in order to carry out the basic functionality of closing the active tab.
Not super hard to implement, the code is already there under the tabs button long press. Perhaps there's some concern about the user accidentally bumping this button and closing their tab, but this is mitigated by the 'undo' toast. Additional countermeasures could include making the button disable-able and/or adding a 'reopen closed tab' button to the action menu, mirroring Ctrl+Shift+T on desktop—I think that a good place to put such a button would be by the history entry, much like the 'Add [bookmark]' button next to the Bookmarks listing