If you're connecting to an e-mail account via IMAP, you may be familiar with unannounced problems with your e-mail that turn out to be quota related. Ideally, your e-mail server will e-mail you before you hit your quota ceiling. However, if someone sends you a file that half the size of your entire quota and your inbox is already 3/4ths full, you're in trouble.
The Display Quota plugin for Thunderbird can help. It adds a percentage bar in the bottom right corner of your Thunderbird window. When you click on your Inbox, look down there, and you'll see a very helpful green bar with a numeric percentage of how much space you're using.
If you right click on the bar, you'll a menu with "Options" at the top. Click on "Options" and you can have Display Quota warn you before the dreaded quota ceiling is hit.
If nothing else, it's good to know how much breathing room you have.