Thunderbird freezes and I have to force quit

I work on a MacBook Pro. I have used Thunderbird for about 15 years. Suddenly, lately, it freezes frequently, with a spinning blue disc or a spinning rainbow disc, and I have to force quit. Would this have anything to do with capacity? I have a lot of folders now of stored emails, after 15 years of using it for business. Could the freezing relate to that? I have tried to delete as much as I can. But the problem continues. I have read through similar complaints here and but I am still not clear what causes it and what to do about it.