Unfortunately, you cannot simply use Add Bookmark to add an email folder bookmark in Netscape, you have to bookmark a message first. Open a message (any message, it really does not matter) in the folder you want to bookmark and add a bookmark for it via Bookmarks | Add Bookmark.
Open the bookmarks window next with Bookmarks | Edit Bookmarks and find the bookmark you just added. Via Edit | Get Info you reach the bookmark's properties.
First, you should give it a meaningful name like "Inbox" or "To-do Items" or "Clarissa Mail". Then you need to edit the bookmark's "Location (URL)". Put the cursor at the very end of the field and delete everything until you reach the name of the bookmarked mailbox, which you should not touch.
For example, the bookmark I created for the message ended in "Mail/Quaxi?id=3A7CABC2.A499376D%40ladedu.com&number=2". After I edited it, the end read merely "Mail/Quaxi".
With that editing done, you can now access the mailbox quickly from anywhere in Netscape via its bookmark.