I believe you. If you wanted to, you can open the network inspector (CTRL-SHIFT-E) and watch it while you try to edit a post here and see if it’s doing anything in particular when the freezes happen. The console (CTRL-SHIFT-K) can also give clues sometimes, but can be overwhelming if you don’t understand the errors (many of them are just false positives).
But neither of those will help if it’s a computer, rather than a Firefox, problem.
Sorry, wasn’t trying to imply anything! Those are the same troubleshooting steps I would use myself (and have used) to resolve similar issues in the past.
In one case, for me, it was a mouse driver causing the stalling — it was polling too frequently and oversaturating the USB bus or something, can’t remember exactly. In another case, a display card issue was causing screen redraw/refresh issues, but only on certain browsers with certain sites. The darndest things can cause mysterious issues like that =/
Cryptic problems like that, and yes, the annoying bloated new interfaces, led to me eventually giving up Windows altogether, and I’m much happier now…
OK, so did your friend reinstall any sort of antivirus on it? If you’re still using McAfee, that in and of itself can be malware-like and cause slowdowns as it’s scanning stuff. McAfee and Norton are especially bad in that regard. The built-in Windows Defender (or whatever it’s called these days) should be good enough and not as much of a resource hog.
If you keep the task manager open in the background (but maybe delay the refresh interval to several seconds), you might be able to tell which process is causing a sudden spike when the system halts like that.
The Event Viewer is the official source for system-level errors: https://learn.microsoft.com/en-us/shows/inside/event-viewer
If any of that is nonsensical, maybe your friend can help again? Got a spare Luigi cap lying around? 