You can simply remove all the ones with the yellow icons. Just be aware when they are gone they will be gone from your server as well.

What about ones like "All Mail?" This seems redundant to me. Entries often appear there as 'unread' even though I marked all mail as read from the Inbox.

If this is a Gmail account, Allmail is redundant and not necessary.

Yeah, delete all mail and see all your mail in that account simply disappear. Not a good idea at all.

Right click the account, select subscribe from the menu and simply unsubscribe from this folder. Until you become Au fait with how Gmail specifically works I strongly suggest you delete nothing as the outcomes you get will be very different to what you expect.

I suggest you begin with this support article https://support.mozilla.org/en-US/kb/thunderbird-and-gmail which will introduce the concepts. However Google is always the canonical source of information about their mail product and specific questions about the Google/Gmail implementation of IMAP should be directed to google.

You might also benefit from selecting the folders google make available to IMAP on this page in your gmail settings https://mail.google.com/mail/u/0/#settings/labels.