I couldn't get JSFiddle to work in Firefox, and it wouldn't load embedded CodePens, either. I put up with it for months (as I don't use either myself), until very recently developer tools also stopped working, and at that point I gave in to the inevitable and created a new profile, which worked fine with all three. I used FF Sync to import my bookmarks and extensions, and it all still worked.
Then I reactivated the fairly basic theme I'd been using for years, and dev tools stopped working again. So I hastily switched back to the default theme, and all is well.