Why does my email not refresh on its own?

Why does my email not refresh on its own?

I have created folders and rules to automate sorting of emails I receive.  Too often, I have noticed that my main view ( https://mail.zoho.com/biz/index.do) does not show me any indication of new email until I go to any particular folder and then all of a sudden the unread count shows up with all the new unread emails. Some time these new emails are several hours old sitting there in the folder but just because I didn't click the folder, I didn't know they were there.

Why is this happening?

I use Firefox and my Zoho mail is always open in a tab along with other open tabs.