Review for IMAP Quota (Free Space) by Swaddy
Rated 5 out of 5 stars
I really liked this addon, big thanks to the developer!
To (always) show the quota on TB 78+, open the Thunderbird Config Editor (First page in settings, just scroll down or maybe see here https://support.mozilla.org/en-US/kb/config-editor) and search for “mail.quota.mainwindow_threshold.show” and set it to 0.
Now you’ll see your quota when you click on an account on the right side of the status bar.
To create your own collections, you must have a Mozilla Add-ons account.