I run four Firefox instances simultaneously, most of the time. No issues to report.
It seems more likely to happen when the profile has been running for a long time (a couple weeks?) and/or using a large amount of RAM.
There's a 60-secish timeout before it gives up and pops that crash report window. I don't think it's a crash per se, just an unresolved file lock or similar. I haven't noticed whether there's any relationship to running multiple profiles. I am almost always running several at a time, and the issue only occurs sometimes. It has no (other) negative side effects, as far as I can tell, but it was unsettling at first.
I'm on macOS also, and I launch from the command line (effectively, I actually have separate launchers for each profile, but they just run a shell script with different arguments).
Honestly, I've been blaming MacOS for it since other apps also crashed at the same time (the first time it was Microsoft Intune, the second time it was Slack - I doubt either uses Firefox internally). I don't recall seeing a Firefox crash on my personal laptop running Linux at any point in the past few years.
My guess is that it's trying to obtain or release a filesystem lock, possibly one that it's lost track of in some trivial way.
I've never seen any damage or inconsistencies in the resulting environment. So I don't think it's a dramatic event, just a safety timer that isn't resolved correctly.
Probably a simple, dumb, but harmless bug.
