Did you know that Mozilla Thunderbird IMAP quota can be displayed at the time at the bottom of Thunderbird's window?
Basically this functionality is built into Thunderbird but the quota is only displayed when IMAP space usage reaches 75% or more.
You can adjust the percentage value by going to about:config (warning, here be dragons) and changing a setting there to a desired percentage.
1. Go to Tools/Options and scroll to the bottom of this page
2. At the bottom there is a button "Config Editor". Click it.
3. Click the "I accept the risk" button.
4. In the search field copy and paste this: "mail.quota.mainwindow_threshold.show"
5. Change the value (percentage) to a desired numerical level.
6. Close the about:config window and restart Thunderbird, and voila, this should be it.