alt Hacker NewsFirefox's translation by default does pop up to interrupt and ask if you want to translate a page that it's detected is in another language. We're just more used to that and it's a more reliable signal that you probably want to run a tool than most.
It's still relatively new in FF and I don't think I've seen anyone complaining about it annoying them with popups, even though it absolutely does throw up an interrupting overlay, especially on mobile.
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You can disable the popup but still invoke the tool manually from the main menu. I reaffirm my previously expressed dissatisfaction with modern software "features" and add that there are plenty of defaults in Firefox that I personally dislike. That includes anything that pops up unsolicited without good reason.
I definitely complain about this one. I can read a few languages, and rarely if ever browse a page in a language I don't understand, so popups with "do you want to translate this" are unwelcome here. It doesn't help that in the first iterations Firefox didn't offer a quick way to turn the whole thing off.