Thunderbird opening on emails 4 years old and now running very slowly.

When I went to open Thunderbird this morning it told me the programme was already running. It certainly was. There were an infinite number of Thunderbirds running! After failing to close them all, I closed down the computer and started it again. Thunderbird was veryslow to load and when it did, the display for my inbox was of emails from four years ago. I strted to scroll down using the mouse but then I noticed it was scrolling by itself. Eventually it reached today and had downloaded some messages. The system is though running very slowly, it takes a while for a message to open and messages which I have left open at the top of the screen for action are not showing subject or content. I operate a complicated system of filing in subfolders of messages I need to keep for reference and the subfolders seem to be ok. I started using Thunderbird many years ago when a IT friend recommended it because, unlike Outlook, Thunderbird has an infinite capacity. I am wondering however if I have taken that too literally? I must have thousands of emails in my IN box going back years, plus a structure of some subfolders which are never used. Is that likely to be why I am having problems and would I be well advised to do a bit of a mega clear out? Advice please from the more tehnically skilled would be much appreciated! Carol