if the account is IMAP and Gmail.

Goto https://mail.google.com/mail/u/0/#settings/labels and ensure the labels are marked to be available in IMAP.

Right click the account in the folder pane and select subscribe. Ensure the labels/folders are subscribed.

Thanks.

Still doesn't show. What do I do?

As you can see, Categories do not have the option to be shown in IMAP. Gmail Categories are filtered "views", kinda like Thunderbird's virtual folders (saved searches).

So, there is no way to group them?

re :So, there is no way to group them? Did you read the comment by Stans ?

Gmail does not allow those filtered searches to be available via Imap. You would have to create your own version of that in Thunderbird.

I do not know what determines a particular email to be classied as Primary, Social, and Promotions. But obviously you do because you use that in Gmail webmail account. I stopped gmail performing those sort of filteres as I missed too many emails.

'Promotions' to me sounds like 'Junk'  :) but maybe different to you if you had emails from eg: M&S that contained offers that are not junk. 'Social' sounds like emails from friends or Facebook etc. 'Primary' sounds like anything else not in the above categories, so basically it's your Inbox less the above.

Options: You could create a 'Social' folder and a Message Filter to detect emails from your friends email addresses, so they are auto filtered to that folder. But this really only works on new mail received into the inbox, so not quite the same thing.

I would look at the 'Saved search' option as Stans advised. You could create a folder that is a saved search, which detects emails determined by your set of parameters on any folder and displays them as a 'Search Folder'. the beauty of this is that it can be updated to include new folders or additional parameters.

• https://support.mozilla.org/en-US/kb/using-saved-searches