Rated 4 out of 5 stars

per Peter el des. 23, 2020 ·

Well, since the extension doesn't work in TB 78, it's good to know that there's built in functionality in TB. The quota is displayed in the status bar. But, by default, only if the usage is more than 75%. If you want to see it all the time, you need modify the option: mail.quota.mainwindow_threshold.show
See https://bugzilla.mozilla.org/show_bug.cgi?id=427066#c1