recently the Gmail team has been working on a structural code change that we'll be rolling out to Firefox 2 and IE 7 users over the coming weeks (with other browsers to follow). You won't notice too many differences to start with, but we're using a new model that enables us to iterate faster and share components (we now use the same rich text editor as Groups and Page Creator, and the Contact Manager can be seen in several Google apps). A few other things you will notice are some new keyboard shortcuts and the ability to bookmark specific messages and email searches. One side effect of this change is that if you're using third-party Gmail extensions, they're likely to stop working. And that is the case for Gmail manager, because this extension no longer shows new messages.

But there is quick fix before the new version:

find your profile folder
browse to the extensions directory
open the folder named {582195F5-92E7-40a0-A127-DB71295901D7}
open the components folder
Open "gmServiceGmail.js" in your favorite text editor (notepad)
Use the find feature to the following text: "?search=inbox&view=tl&start=0&init=1" (without the quotes)
Append "&ui=1" to that text (again, without the quotes)
Save the file, and then restart Firefox! It should now show your unread messages!

Hope that helps someone!