Probably. Never seen anything like that on Windows or macOS.

How can this happen when Linux package managers don't auto-update (without help)? I mean if you don't want Firefox updated while you are using it you just just don't update it. If your distro has some sort of auto-update by default (do any do this?, I thought available update notifications was the standard) then disable it and get control of that back. Auto-updating packages is an anti-pattern (ie. bad).

… it doesn't, and I've just run `pacman -S` to check. (& it didn't.)

(It does cause some tabs to message that a browser restart is required. Even so … this is really the first I hit this, as I'm usually doing updates when it is opportunistic to do so, such as right before a reboot (so any kernel updates happen).)

It can also happen if you have two copies of 'stock' Firefox running (with different profiles) and one of them runs the update.