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.
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.
The folders' tree structure can be flattened via Advanced Account Settings and setting the IMAP Server Directory to null, then restarting Thunderbird. TODO: was not able to find this setting in TB 17.0 ESR.
Sample script to be run in the desktop environment:
export NSPR_LOG_FILE=/tmp/thunderbird.imap.$(date Y-%m-%d@%H:%M:%S).log
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.
In case of trouble with "too many simultaneous connections" (common with firstname.lastname@example.org), reduce the "Maximum number of server connections to cache" (TODO: presumably on all clients); the default value is 5. Account settings > Server Settings > Advanced.
Thunderbird as a Gmail client: http://kb.mozillazine.org/Gmail