It happens within Firefox, without rhyme or reason that I could discern. I think it’s more common if I open a link from another app, then switch away from Firefox while the page loads in the background; or when returning to FF and it tries to reload a previously suspended tab. Firefox is allowed to load stuff on my phone in the background even when battery saving is on (which it always is), but I guess FF might get into a fight with the system over available memory, with the system killing one of FF’s processes/threads. Wouldn’t explain it happening on the desktop, though.

If this is such a percentage issue I’d there big open about it?

It happens pretty rarely for me, and I have no idea how it could be reproduced. So, I already barely ever open issues (by principle), and certainly won’t report this. Idk about others — wouldn’t be surprised if there’s a bug about this among the many thousands of issues that Mozilla has.