Yes, most definitely swap memory. As a retired programmer, I have seen a lot of mem-leaks, but this one beats almost all. Interestingly, the resident programs (Scrivener and Messages) still worked and with no lag, but a new app could not be opened. Killed it and everything worked fine, although I then rebooted just to clean everything up.
Wonder if anyone has seen something like this on other than Firefox. On a new M1, I mean.