If your Mozilla Thunderbird is slow to open the inbox, or any other folder for that matter, there are two things that you can do to try to fix this problem.
Step #1: The first thing is to try to compact the folder. This should help if you have a large inbox. If you can’t even get to this step, or if it is just too slow, try step #2.
Step #2: The .msf “index” file is corrupted. If this file gets corrupted, you could see a slow initial loading of your inbox, messages will not open, you can not delete messages, etc. You may also see a message in the status windows when you open Thunderbird that says something about rebuilting index. The solution to this is to first close out of Thunderbird, then delete the inbox.msf file. If this is for a different folder name, you will see “folder name.msf” file corresponding to that folder. This file is located in your profile directory. For me this was: “C:\Documents and Settings\Dave\Application Data\Thunderbird\Profiles\default\svxnp81c.slt\Mail\systembash.com”
But this varies because of the random profile string and your account profile name.
Delete this file. Make sure you leave the real inbox file alone – this is a file that is named “inbox” (or your folder name) without a file extension.
Re-open Thunderbird – the index will automatically be rebuilt and you should now have normal access to your inbox!