...

Introduction

TBC

Troubleshooting

Attachments do not open

Try re-initialising Thunderbird's configuration of how to open them by Preferences→Attachments then selecting the affected "Content type" expanding its Action drop down list and selecting "Delete Action".  Try to open the attachment again; this should prompt you to browse for the program Thunderbird will open it with.

Greyed out folders

Folders are shown in grey italics when they have the \NoSelect flag.  Normally this means they can contain subfolders, but not messages.  A good example is the "[Gmail]" folder on Gmail accounts.

...

Missing folders

May not be subscribed.  Investigate by opening any folder's context menu and fully expanding the list to show which folders are not subscribed.

In case the IMAP server is Cyrus, further investigation procedures can be found via Email and messaging#Cyrus.

Too many simultaneous connections

...