Are you referring to the unread emails count? You can mark emails as read in bulk by right-clicking the folder and selecting the "Mark Folder Read" option. If you are referring to removing the count regardless of the status of read emails, then from my understanding this is not possible.

See https://support.mozilla.org/en-US/questions/1426787#answer-1609463

How do I get rid of the number count next to my inbox -

It reads: Inbox, 4 unread messages, 4,375 total messages all on one line

Can someone help me with this?

The total number can be hidden by clicking the ... icon next to the New Message button. The number of unread can be hidden with the method in my first reply.