SelectTools>Account Settingsfrom the menu.
Select theSynchronization & Storagecategory for the desired IMAP account.
CheckKeep messages for this account on this computer.
CheckDownloadfor theInboxfolder and any others you want available offline.
How to Access IMAP Inbox Offline in Mozilla Thunderbird
How to access your IMAP emails in Mozilla Thunderbird, Mozilla SeaMonkey, or Netscape even when you're offline.
