I have cleared cookies, logged out and back in, and disabled ublock origin. I have no other add-ons. Running latest version of Firefox. It works in chrome and edge with ublock running. Driving me crazy having to edit the answers just to see what they are.
Additionally, spoiler tags on other Stack Exchange sites work normally.
Spoiler tags on other exchanges look like this in my browser:
Spoiler tags on the Puzzling exchange look like this: