That’s all fine and good, yes (you’re comparing mac to ios) . . . . but it still doesn’t explain why tabs don’t close when clicking the ‘X’ on the ios FF tab bar. To use your analogy, it’s not because of the engine “limitations” - it’s a bells/whistles and outside (the engine) function that other ios based browsers don’t have a problem with.
The odd thing is that it works perfectly fine every time on my 2021 12.9 and Mini 6.