Firefox 15 was just released this week and I have been using it since the beta began and as far as I am concerned it is the best browser for development purposes. Let me highlight a couple of reasons why.
Working with CSS
A lot of times when developing a web site or web application, getting the "look" just right requires quite the effort. Up to now one of the best tools for working with CSS in Firefox was Firebug. However with the latest Firefox, you can just right-click on any element on a page and select "Inspect Element (Q)" and you get to see all the properties of the element and adjust them on the fly and see what effect your changes. Now this has been there for a while, but with version 15, in the bottom-right corner, you now get visual tool showing you the overall size of the element and the size of each of its properties (borders, padding and margins). Quite useful, here is a sample:
Testing for Mobile
As we go along in 2012 and beyond, a good portion of work will require us to make sure a site is mobile-ready. So if you are doing Responsive Design (using CSS Media Queries), you can now test your design in Firefox 15 without the need of having various physical devices. Simply navigate to the site and select from the Firefox menu, "Web Developer > Responsive Design View" and you will be able to see how your site looks based on some predefined screen resolutions that are available.
So happy developing using Firefox 15!