Do you know that Thunderbird has a statusbar to show the quota used for an IMAP account?
It's an hidden feature and you need to manually change some configurations to show it always.
This addon let you choose how and when show the IMAP quota status bar.
Development discontinued. More info here
Documentation & support
Bugs
To send a bug report use github.
Future improvements
Check the issue tracker for planned features.
How does it work?
After installing, open the addon preference panel to choose when and how to show the IMAP quota statusbar.
The statusbar is shown only when is selected a folder of an IMAP account.
Thanks to Dillinger, for updating this addon for the Thunderbird 60+ compatibility, and to BabelZilla for the translation support.