That is Firefox standalone behavior when it detects its files have been changed and differ from the ones loaded by the current instance. In theory, what snap is doing avoid changing files from a program while it is running.

Now, I don't know how your setup looks like, but I don't think anything is distributed as snaps by default on Arch. At least AFAIK its mostly an Ubuntu & derivatives thing.

Well, it was more broken in more interesting ways before they implemented the forced restart