IMAP mails disapear while offline

Hello Everyone, I am using Portable Thunderbird 3.0 with IMAP setting for Gmail. In my Sync and storage option "store mails on this computer" option is checked. While i am online Thuderbird downloads all the emails and everything works fine, but as soon as i go offline, all the downloaded emails as well as gmail folders disappear and there is not even 1 message left in inbox. However thats not the case with my another POP3 account with Hotmail and i can read my emails offline for POP3.

Please Advise what seems to be wrong.

Thanks

Hitesh