You can update Thunderbird later (which I definitely recommend), but first you need to deal with the Inbox full error. Inbox full error '''1.''' [https://mzl.la/2lBfidM#w_backing-up-a-profile Backup the Thunderbird Profile folder]. '''2.''' Create a new sub-folder under Local Folders. Move '''all''' of the messages and folders currently in your Inbox to that new subfolder. '''3. '''Confirm that you have made the recommended backup of your Profile. '''4.''' Find & open your Thunderbird Profile folder: Select Help: Troubleshooting Information from the Thunderbird menu (either from the menu button , on the top right, or the normal menu at the top of the window.) Under "Application Basics", beside "Profile Folder", click the "Show Profile" button.  This will open a window showing the contents of your Thunderbird Profile folder. '''5.''' Quit Thunderbird, but keep that window open showing your Thunderbird Profile folder. '''6.''' Delete the files called '''Inbox''' and '''Inbox.msf''' for that account. If you are using IMAP, you will find them under '''ImapMail''', and a subfolder for your e-mail provider. If you are using POP, you will find them under '''Mail''', and a subfolder for your e-mail provider. '''7.''' Start Thunderbird. You will now have a clean new Inbox for that account. '''8.''' Read and follow the advice in the article [http://kb.mozillazine.org/Keep_it_working_-_Thunderbird Keep it working - Thunderbird]

Once you have done all that, then: '''1.''' [https://mzl.la/2lBfidM#w_backing-up-a-profile Backup the Thunderbird Profile folder] again. '''2.''' [https://www.mozilla.org/thunderbird/ Download & install the latest version of Thunderbird]