Did you choose POP or IMAP? POP can only see the Inbox. You will need IMAP to see your other folders on the server.

I believe it must be IMAP, because I do have other folders (other than the inbox) showing. Three folders - other than the inbox - showed up. And two did not. It is very random and strange.

What are the other folders that did show up? Thunderbird expects certain folders to exist (e.g. sent, trash, junk) and so looks for them. It cannot know about user-defined folders so may need you to subscribe to them.

Right-click your account, select "subscribe" and make sure the missing folders' checkboxes are ticked.

Then right-click the account, select "settings" and then "synchronisation" and again make sure your missing folders are included. I'd recommend setting Thunderbird to "store messages on this computer" since this makes searching so much faster and more pleasant to work with.

Ah okay that is what did it.

There were user-defined folders (folders I had created within my fastmail account, such as one for personal development related correspondence, and another for employment applications) that both showed up no problem.

I am not sure why the others did not show up, but the "manage folder subscriptions" tab under my account did show the other folders in my fastmail account, and I was able to check the boxes next to the two I wanted to show up in Thunderbird, and they immediately showed up.

Thank you very much for your very prompt replies and assistance!