I'm using ThunderBird 3 for the first time and it has done synced my mailbox through IMAP. Can anyone tell me how to remove an email's label in Gmail using ThunderBird?
You should consider using something newer than TB v3. It's quite old. I believe v10 or v11 is current. :)
Gmail's labels don't really correspond to anything sensible in IMAP so we use folders as a workaround. The big difference though is that a message can have multiple gmail labels but in IMAP can only exist in one folder at a time. The end-result on your computer is that for any message that has more than one label in gmail, your Thunderbird will make and keep A COPY of that message in each of the corresponding folders. To remove a gmail label from a msg is the same as deleting one of these copies from thunderbird.
Be careful, I'm not sure how you tell if you're deleting the last 'copy' and so actually deleting the message itself permanently.