profiler VS old-reddit-redirect

Probably your best bet is to use the Firefox Profiler once it starts hogging the CPU to try to see what it's doing. I'd be happy to take a look at the result although I don't have a lot of experience at diagnosing performance problems; hopefully the profile will make it obvious what's going on.

I tried using the https://profiler.firefox.com/ and running with:

You can try to reproduce while taking a profile using the Firefox profiler, then share it in the #perf:mozilla.org room on Mozilla's Matrix instance.

Run a profiling session for ~30 sec when you notice something is running slower than you expect, see https://profiler.firefox.com/. Share results here if you want help interpreting them.

Next time, send a SIGABRT to the main process (kill -6 $(pidof firefox)) and use the resulting backtrace (you will find a link to the crash report in about:crashes) to file a bug. A performance profile could be useful, but you'll need to know first what to capture. This seems like a graphics issue, so try the Graphics preset. Share a link to your report.

https://profiler.firefox.com will be useful if you can capture and share it to developers.

If you can reproduce it in safe mode and a clean profile, capture a performance profile using the clean profile while trying to load a problematic webpage, don't remove any information from it, and share it here (although it's typically not easy for users to analyse them), or file a bug with it attached.

Next time, capture a performance profile using the Graphics preset, and kill the browser by entering kill -6 $(pidof firefox) in a shell. This will create a bug report (unless debian disables the crash reporter) that would indicate where Firefox is stuck at. You might need to use an official build to get useful results.

Sorry this doesn't answer the question you asked, but why not use the old reddit redirect extension?

Has been tested with other extensions disabled, but for context, I use Old Reddit Redirect and RES

I have a browser addon installed (chrome/firefox, link for anyone interested) that redirects to old reddit even when logged out e.g. in private windows, and it works perfectly 👌

I just use this extension to redirect it, also have RES setup along with Masstagger. On the phone I use RIF.

This breaks the Old Reddit Redirect extension, which uses this redirect mechanism to gracefully redirect direct links to galleries to the full old reddit post. See https://github.com/tom-james-watson/old-reddit-redirect/issues/72 for more context.