Hello

if you don't give some information about your setup, it's difficult to help you. If you have an IMAP account, the messages are stored by your mail provider and the limitation is configured by the provider. If it's a POP3 account, the messages are downloaded from the provider as they are incoming and the limit should be your hard disk.

my account is an imap account, how do i change to POP3 to be able to store more emails?

First, you cannot change an IMAP account to a POP account. If POP is what you want, you need to create a new account alongside your IMAP account and then shut down the IMAP account when you're content with new account. HOWEVER, I encourage you to review the storage issue first. For example, are you getting messages from Thunderbird? Or is your disk drive nearly full? You never told us about that.

David is right as it's not so easy to change from IMAP to POP and you should do it only in the specific case where your provider limits your Imap mailbox to a small size and your hard disk can store a lot more. But there are other cases, such as indeed your hard disk is full, or your provider limits the size of sync data, or you have in your Thunderbird account sync and storage parameters set to sync only the 30 last days. So more info is needed to understand if the limit is a global size or a period, and what are the error messages exactly.

There is a free, very small, utility called WizTree at this URL: diskanalyzer dot com. Download and run that and it will quickly show you how your disk is being used. You may find huge areas that can be removed. Doing that is a good sanity check on the drive. If it's nearly full, then your real solution may be to get an external drive and switch your email accounts to the other drive.

no my disk drive isnt full and has plenty of storage. Iys just the thunderbird email platform telling me my messages are full the bar down the bottom is constantly red or at around 90% full

that's your SERVER mail quota.