Are the problem images just static JPG/PNG/GIF, or are they GIF-V, which are really videos? Or maybe something using a canvas?

Just plain old ordinary JPG. I don't know how to check whether there is a canvas. You could look here as it has just done it again GumTree the page loads alright, but when I click the right arrow it seizes.

These GumTree pages seem to use 100% of both processors, use 60% of RAM (4GB/3.2GB) and 80% of swap (1.1GB). Maybe the problem is something to do with that. However Chromium and Opera both cope with it. Maybe the memory management is better with those two browsers.

Any difference if you use a Private window with tracking protection enabled (it is enabled by default) to limit the third party ad and tracking scripts in the page?

Immediately seizes up solid.

Mysterious. Could you try:

New Profile Test

This takes about 3 minutes, plus the time to test your problem site(s).

Inside Firefox, type or paste about:profiles in the address bar and press Enter/Return to load it.

Click the "Create a New Profile" button, then click Next. Assign a name like May2019, ignore the option to relocate the profile folder, and click the Finish button.

After creating the profile, scroll down to it and click the Launch profile in new browser button.

Firefox should open a new window that looks like a brand new, uncustomized installation. (Your existing Firefox window(s) should not be affected.) Please ignore any tabs enticing you to connect to a Sync account or to activate extensions found on your system so we can get a clean test.

Do the problem slide shows work any better in the new profile?

When you are done with the experiment, you can close the extra window without affecting your regular Firefox profile. Assuming the process isolation is working as expected.

Worth trying, but with the new profile it still seizes up.