It appears that 62.0.2 (not 62.0-2) was released today by Mozilla. When it is available in snap, you may want to try it to see if it fixes your problem.

gnome-logs shows that AppArmor is denying something with snap, which is how I've installed Firefox. Firefox had not been updated in a while, so something else was changed during one of the recent PureOS updates. I'm not new to the concept of reverse firewalls, but I'm a complete newbie to AppArmor, so will have to dig in.

From gnome-logs:

audit: type=1400 audit(1537589776.280:85): apparmor="DENIED" operation="ptrace" profile="/usr/lib/snapd/snap-confine" pid=28612 comm="snap-confine" requested_mask="read" denied_mask="read" peer="unconfined"

I am glad you isolated the problem. Having been a developer in a previous life, I was only thinking that maybe Mozilla fixed something that might have coincided with the updates.

In any case, I have be struggling mightily with apparmor on Firefox, because I took away its universal reads. (Part of that is ignorance of apparmor; I too am new to it.) Insodoing, I noticed there are many Firefox profiles circulating. Consequently, my suggestion would be to compare the latest apparmor profile in snap (github?) with what you have on your system. It may very well have that 'read' permission. It seems to me that snap would need to other, specific processes.

As of today 2018-10-08, after intervening system updates, this issue persists. In the meantime I had to install an Xubuntu VM in Boxes, and then install Firefox within the VM.