I'm running the Ubuntu snap version of Firefox and recently I've had a problem where the more files I download the more sluggish downloads become. When I download a file now, even a few KB, Firefox becomes unresponsive for a few seconds, including video in other windows freezing. For longer downloads, Firefox is unresponsive the entire time. During downloads Firefox uses 1 core 100% in htop and about:performance shows "Firefox" using 100%

Clearing all data in settings doesn't help

Clearing the list of downloaded files doesn't help.

Deleting the profile fixes it, but the problem comes back as I download files.

I have a policies.json file that sets some privacy related settings and installs 3 addons (uBlock0@raymondhill.net, treestyletab@piro.sakura.ne.jp, @testpilot-containers).

I also have a hosts file that blocks all mozilla domains (except addons.mozilla.org)

I have a fast CPU, tons of ram and a nvme drive.

I'm not really sure how to debug this?