If you are using 'View > Folders > Unified Then you cannot add a folder as a subfolder to a virtual unified 'Inbox' You need to click on the chevron > located to the left of the Unified 'Inbox' to expand and see the individual Inbox folders. Those individual account Inbox folders will have the account name not the name Inbox. Right click on the account name and select 'New Folder' That should allow you to add a new subfolder to that individual account Inbox.

IF you are using 'View' > 'Folders' > 'All' The mail accounts should be listed with their own set of folders. Right click on the 'Inbox' and select 'New Folder'

It would be useful to know what happens when you right click on Inbox. Is the 'New folder' option greyed out ? Or is the option available and you enter details, but 'Create folder' is not creating folder ?

Are you talking about an Imap mail account ? right click on imap mail account name folder and select 'Subscribe' click on 'Refresh' Do you see the new folder in the list ? If yes, select it and clickon 'Subscribe' clickon 'OK'

Sear Toad Hall Didn't work. I use New Folder, enter e.g. Proz, click Create and ...nothing! No Proz sub-folder anywhere.

Can you please supply some information. Q1: Are you using 'View' > 'Folders' > 'All' OR 'Unified' ? Q2: Are you trying to create that new folder as a subfolder in a POP account Inbox Or an Imap account Inbox or in 'Local Folders' account Inbox?

Edited : I've also had another idea....I've come across some people who also had issues with creating folder/subfolders. It turned out there was a weird occurance where a spam url was causing a problem. But for some people they could not even type anything. See bug report: https://bugzilla.mozilla.org/show_bug.cgi?id=1911225#c12

Please check the following: In Account Settings: For every mail account and Local Folders accounts check the following Junk Settings:

select checkbox 'Enable adaptive junk mail controls for this account' select the checkbox 'Move new junk messages to' and select where to move them - examples below. The important thing is this - if its an imap account use the 'Other' option.

• Select Junk Settings' for pop gmail account
• Select checkbox 'Move new junk messages to'
• Select 'Junk folder on - gmail email address pop account.

• Select Junk Settings' for imap gmail account
• Select checkbox 'Move new junk messages to'
• Select 'Other' and - imap gmail email address/account.

• Select Junk Settings' for 'Local Folders' mail account
• Select checkbox 'Move new junk messages to'
• Select 'Junk folder on - Local Folders

Then restart Thunderbird Attempt to create a subfolder in Inbox.