I cannot access a messages in particular folder on the sidebar
This folder got duplicated a couple of weeks ago and now I cannot open either. If I do a mail search it will reveal file(s) within but I cannot delete or browse that folder
Firefox doesn't do email, it's a web browser.
If you are using Firefox to access your mail, you are using "web-mail". You need to seek support from your service provider or a forum for that service.