Hmm, that kind of freeze is unusual.

Not site-specific:

In the past users with huge collections of fonts have sometimes seen that on sites that use downloadable fonts.

Could you disable downloadable fonts as a test and see whether the problem goes away after the next restart? Here's how:

(1) In a new tab, type or paste about:config in the address bar and press Enter/Return. Click the button promising to be careful.

(2) In the search box above the list, type or paste gfx and pause while the list is filtered

(3) Double-click the gfx.downloadable_fonts.enabled preference to switch it from true to false

Obviously not a solution, but any difference?

Site-specific:

Does it matter what kind of media is on the page you visit, such as animated GIF / MP4 vs. a static PNG?

That non site-specific suggestion -appears- to have solved the issue.. I changed the entry in the config file, restarted and tested by opening Imgur. I could not get the freeze to happen.

Regarding your second point, it only seems to trigger when the site itself is loaded, ie the Imgur UI and menus and such, whereas a direct link to the image itself, surrounded by the default grey of Firefox, does not seem to trigger it. This would make sense if downloading the fonts is what is causing this.

Thank you kindly for fixing the issue, if it occurs again, I'll update this thread.

Well, that's a useful diagnostic but definitely not an ideal solution. Hopefully someone else has a way to speed up the font analysis so you can still use website fonts where they're critical.