I have seen my fair share of the add-ons for Firefox, or Greasemonkey scripts that promise to integrate Gmail with just about anything and everything. They all have little flaws in them though, that keep me from wanting to use them all the time.
One new add-on that might change my mind is Integrated GMail, an experimental extension that does a fine job at the previously mentioned task. It allows you to make the inbox collapsible and loads your choice of Google Calendar, Google Reader, Google Notebook and Google Groups under it (they are also in collapsible boxes).
It was also recently featured on my list of 9 Google Reader Add-ons for Firefox via Mitchelaneous.com.
If your are inbox minimalist, enable minimalist mode to clean up the inbox even more. This extension is well worth checking out. You can pick it up on the Firefox Add-ons web site. Once you do, let me know how it stacks up against other favorite Gmail tools of yours for Firefox.