I reproduced it just now after reading your post.
- Clicked "back"
- "Document expired"
- Clicked "reload"
- "To display this page, Firefox must send information ...."
I'm also seeing a lot of "Cloudflare checking your browser" and "Tick the I'm not a robot box". Not just occasionally but roughly every 15 minutes or so. Not sure why they don't set a cookie and save themselves some resources and me some annoyance.