Around version 73 Mozilla changed extension support in Thunderbird.
That broke the quota extension and the developer Arnaud Abelard got fed up with it as so many.
Now a user ( Peter
) of the old extension has posted an alternative on the developers extension page
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:
Just open the Config Editor ( Tools -> General -> (at the bottom) Config Editor )
and search for
and then change the default value from 75 to what ever you want below that to always or after your value show it in the status bar..