This question --> https://support.mozilla.org/en-US/questions/836545?s=Keep+getting+We+noticed+you+may+have+signed+in+or+signed+out+in+another+window.+Click+OK+to+reload+your+page.+everytime+I+start+firefox
is locked because the original question is over a year old. Advice given on prior Firefox versions may vary from advice given on current Firefox versions. Posting in more current similar questions with the same/similar topic may be allowed.

Look for another similar thread/question or post a new question giving all of your details.

If this reply solves your problem, please click "Solved It" next to this reply when signed-in to the forum.

I understand what you are saying, but comments were coming in as recently as last week. It should not be locked, it should continue to be responded to and fixed. I still have the problem and the only solution for me is to revert to 12

Many of the threads/questions on that topic are a year old or more and have been locked. Too frequently, users never return to their questions to mark them solved, leaving us without knowledge as to whether any of the posted suggestions solved their problem.

Please post a new question.

Is this on Yahoo? There is a very long thread here in the forum on this problem.

One possible cause is your proxy settings. If you check this deeply buried dialog, you may be able to correct it:

orange Firefox button or classic Tools menu > Options > Advanced > Network

Click the "Settings" button and choose either "No Proxy" or "Use System Settings" option, and OK the change. See whether that helps.

More suggestions here: https://support.mozilla.org/en-US/questions/928922#answer-339570