How big is the Junk folder?

Make hidden folders visible.

• http://kb.mozillazine.org/Show_hidden_files_and_folders

In Thunderbird

• Help > Troubleshooting Information
• Click on 'Show Folders' button

A new window opens

• Close Thunderbird now
• If Pop account click on 'Mail' folder
• If IMAP account click on 'ImapMail' folder
• click on mail account name
• Look for 'Junk' file (not Junk.msf) what Size is that file?

Just in case this could be an indexing issue - delete the Junk.msf file - this is just an indexing file, so deleting it will force TB to create a new one.

Then restart Thunderbird. Try to select 'Junk'.

Report back on results of Junk file size and whether creating a new index file showed up emails.

Did all that, and thanks for the response.

File was 111kb

But when I restarted Thunderbird, and eventually the .msf file was recreated (now 0kb), I still cannot open the Junk folder...which has gone from showing 16 messages to over 200 in the time of trying the solution

Is this an IMAP mail account Junk folder? If yes: The junk.msf file should be more than 0kb as it is designed to read the Junk file. Zero implies it isn't reading the Junk file. check to see if you have subscribed to see Junk folder. check to see if the subscribed folder is selected to synchronise.

• https://support.mozilla.org/en-US/kb/imap-synchronization#w_configuring-synchronization-and-disk-space-usage