I don't think it's warranted in this case. Since we're on Lemmy, we can assume high technical literacy as the average user here is significantly more technologically aware than that of our generic Reddit counterpart, so they are almost certainly at least familiar with the concept of browser add-ons. Even if Firefox isn't your browser of choice, you do know how add-ons work and how they are distributed to end users, right? You don't need to know the details of this specific set of events to know that the OP meant the add-on marketplace.

Don't be a dick; you know from context they are talking about the Firefox add-on marketplace. It's not like Mozilla calls it anything in particular, anyway, so yes, "web store" works.