Please post a screenshot of what Thunderbird shows. https://support.mozilla.org/kb/how-do-i-create-screenshot-my-problem

In any case, your quota is imposed by the server, regardless of what Thunderbird shows.

it shows even less than 5GB

4,294,967,295 kB = 4.2 TB If this number is correct, I wouldn't complain. But how you managed to accumulate 3 TB of mail on the server is a miracle to me.

it must be in bytes because I have about 3GB in mailbox and 4GB was the initial size of that mailbox. More than a year ago it grew to 5 and now it's above 7GB. I didn't notice this until I got the orange warning in the status bar. Later I deleted a lot of mails so now I'm below the 75% threshold

It is my understanding the numbers are reported by the server. For any inconsistencies you should check with your email provider.

so there is now way how to force update of this value from the server?

this is current status on the server - 36% out of 8GB now - the mailbox is still growing :)

I suppose it's part of the IMAP protocol. Talk to your email provider.

OK, thank you