If you account is IMAP, it will be your usage of the accounts quota or the amount of space your provider offers on their servers to store your mail. This allocation varies from provider to provider and can be as little a a few megabytes to gigabytes.

Thee is also the issue that not all providers IMAP servers provide accurate information to mail clients as to quota usage. Most do but some do lie. So check the figure against your providers web mail to determine the accuracy of the value supplied.

Try compacting your folders from the file menu. This allows deleted mail to be purged from the server (but only if the trash is emptied trash is still occupying space)

Usually when you get to 100% you get no more mail and senders get bounce messages that your account is full and not accepting more mail.

Thanks. Your input is most helpful. They are IMAP accounts, and I am just now noticing that it's a different percent for each IMAP account. My fear had been that the Thunderbird client would self-destruct or something.

In addition to good advice from Matt: Are you using a gmail imap mail account ? When you delete an email it must be put into the subscribed gmail imap trash folder. Gmail auto deletes any emails over 30 days old from that server trash folder. They also delete it from the 'All Mail' folder when removed from trash. So quota would be reduced as email has been removed from the 'All Mail' folder.

If you delete an email, but do not put it in the gmail server trash folder, then only the label eg: 'Inbox' is removed. So the email is auto archived by gmail and it is still in the 'All Mail' folder. This means your quota is not reduced as the email is still on server.