I am having this same issue and have been for a few months throughout several updates of Firefox and my macOS, all other browsers work fine. Has anyone found a solution?
This should not be happening. Please refresh your browser. Also try in private mode in firefox - if this works - then clean out firefox data (cookies, cache) and try again.
Are you logged in (then what type of user? just personal or any orgs)? Also, can you send me URL when you have an issue? I would like to be able to reproduce the symptom - so far - wasn't successful.