I probably don't have the answer to this, but in order to get more eyeballs on this question, may I suggest that you post what operating system you have (e.g. Windows 10, or macOS 11), and also what version of Firefox. You can get that info by going to "Help -> About Firefox" on the menus.

Thanks for the suggestion. I have an HP laptop running Windows 10, FF version 133.0 (64-bit)

Screenshot of the error would help others to know what to look for and what could be done to fix it or find the cause of it.

I tried to get a screenshot but it didn't work. It's just the FF logo with the letters under it.

It sounds like it's possibly a downloaded .htm or .html file. Questions:

(1) Do your downloads normally go to your desktop?

(2) When you see one, if you check the Downloads list in Firefox (Ctrl+J), can you trace it back to a particular site? If it seems to be related to a moz-safe-about download, they are a bit of a mystery, but seem to be triggered on some sites possibly by ads.