Says Joe Drew, who's working on Azure:
Firefox 4's graphics performance is great...We're not content with "great," though, and our investigations into how to make drawing even faster have revealed that some of our choices in Gecko's graphics engine aren't optimal for performance.
Naturally, Azure is designed to improve the performance. It also lays the groundwork for future work in separating Mozilla processes into different memory compartments, a long-running project called Electrolysis. The compartmentalization has the potential to improve security and performance, but it requires extensive retooling of basic parts of the browser.
The browser market has become hotly competitive, with Google's Chrome stealing away share of usage once held by Mozilla's Firefox and IE9 finally giving Microsoft a competitive browser again. Responding to the challenge, Mozilla has put Firefox on a Chrome-esque development fast track to try to get new features into people's hands faster.
And, more to the point, all the browser makers are trying furiously to court Web developers with features to make their lives easier. Web developers are the ones who ensure a Web site or Web application works on multiple browsers and takes advantages of new features.