start with Right clicking the deleted/trash folder and selecting empty trash. Then the file menu and compact and see if that helps.

Those two things will mostly clear up "hidden" usage. The next step if move some mail from account folders to local folders as local folders is limited only by the size of your hard disk. IMAP account sizes are limited by the space allocated by your mail provider. If you use Yahoo,Outlook o google I think all three offer increased storage for a fee. But that is well outside of the purview of Thunderbird support.

I followed your suggestion and no longer have the "99%" in the bottom bar so I assume this has helped. I am now trying to find the "window" that shows me "What Percentage" I am using to confirm the problem has been solved. Thanks in advance for your help.

Right-click the Inbox, Properties, and see if the Quota tab looks like the attached picture.

I followed your suggestion and no longer have the "99%" in the bottom bar so I assume this has helped. I am now trying to find the "window" that shows me "What Percentage" I am using to confirm the problem has been solved. Thanks in advance for your help.

Personally I would look on your providers web mail. The actually quota information is provided to Thunderbird by the mail server, but sometimes the information supplied is not actually correct (I assume server software bugs might be the issue). As Thunderbird is not limiting you in any way, and never will based on quota, only reporting what it is given for your information. Verifying what is reported against the source is a good idea at least once to be sure what you are being told by the software is accurate.