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.

I was starting to doubt my memory of this happening in Firefox Android, but I'm happy to report that I've gotten that issue just now. Opened a pic from a Lemmy comment in FF because it wasn't showing in the app, and the tab was sitting there doing nothing. Had to close it and open another one. So you can judge how pressing this problem is for me, by it occurring once in 23 days (likely more) — while I was browsing on the phone quite a bit and opening images from Lemmy regularly due to the ISP's shenanigans. But it does happen.