Did you delete first and then compact afterwards? I ask this because compacting removes 'marked as deleted' emails, so has to be done after. Also, after emptying the trash and junk folders, they would need compacting.

Is this a pop or Imap mail account?

I have compacted then deleted and vice versa. And I've compacted junk and trash as well. Still no change.

It is a pop account

How many emails do you have in your Inbox?

Please move all emails in Inbox to other suitable folders to organise them. Create new folders if required. It is generally advised to use the Inbox as an Inbox for new mail and not as a general storage for old mail, so it should be low in email numbers and compacted more frequently then other folders as the Inbox gets more activity.

Set up Archive Options to archive by month and retain structure folder.

• see info: https://support.mozilla.org/en-US/kb/archived-messages

Then archive anything older than a month. You will still have access to archived emails as they are stored under the 'Archives' folder in your Profile, but it will reduce the file size.