I found Firefox (latest version) unusable on GumTree, as clicking on the left/right browsing arrows on images hammered the processor then seized up the desktop. Opera was almost as bad, so I used Chrome which was usable but struggled. No useful answer from Mozilla or GumTree forums.
I then spotted this somewhere more-or-less by chance:
gsettings set org.freedesktop.Tracker.Miner.Files crawling-interval -2
gsettings set org.freedesktop.Tracker.Miner.Files enable-monitors false
It seems to have fixed it. I believe this is to do with file indexing, is it likely that it could be relevant? It’s possible there could have been some change to Firefox or GumTree since I last used them of course.