A search for Firefox with Twitter turns up a few hits from earlier this month.  

There is a website where you can download the app.  

You can type @food or #bart in the address bar of the browser and be transported to Twitter where it shows you the profile for the user "food" or a search for the term "bart." 

Love to know: 1. Who did it. 2. How it came about. 3. Where it's going. 

This was announced along with their deal with Photobucket. Pretty significant, for both Twitter and Firefox. Imagine if Facebook launched a browser. Same idea.  

PS: There's a help page for this product. 

PPS: There's a Storify thread from June on this topic.