You probably already know how to find unread messages in Gmail (search is:unread in:inbox, but if you want quick, bookmarkable access to only those unread messages in your inbox, CNET has a simple suggestion.
The video above walks through the basic idea. They suggest not just adding a bookmark to the is:unread in:inbox URL (which, if you're a regular old Gmail user, is http://mail.google.com/mail/?&shva=1#search/is%3Aunread+in%3Ainbox), but also setting up a keyword bookmark for that URL so you can instantly open your unread messages in a couple of keystrokes.
I love the last part, but then I think keyword bookmarking is one of the best features in Firefox. Either way, this simple bookmark could be a very useful one if you have trouble keeping your unread messages at "zero".