Mozilla Thunderbird shows the error message: “There is not enough disk space to download new messages. Try deleting old mail, emptying the Trash folder, and compacting your mail folders, and then try again.”
You verify and there is plenty of disk space.
This is happening because Mozilla Thunderbird is a 32 bit application and your inbox folder has more than 2GB of email.
To solve your problem, create a new folder and then copy your email to it.
Delete your emails from the inbox and then run Compact Folders.
Close Mozilla Thunderbird and open it again. Your email should be downloading.