Are you using the most recent update. Version 62.0
It will be either a cookie problem or an add blocker problem.
Firefox is my main Browser but I have to use Chrome most of the time as Firefox reacts badly to PCA if you are using add blockers.
Most of the time I cant sign in to this forum because of it.
But mainly use Firefox for searching and chrome banking etc.
I actually get a box when deleting Firefox which says Firefox did not shut down properly tick here to notify Firefox.
And thats a wast of time.