This is obviously a response to Google Chrome releasing a new version every six weeks. Chrome is eating Firefox’s lunch. Maybe not in overall marketshare, but Chrome is definitely faster. In the browser market, faster is better. People are noticing this.
Also, developers are shifting to Chrome, and I would argue that developers were a big force in pushing adoption of Firefox. Chrome is helping itself in this area by having native dev tools and focusing on improving those tools.
Firefox 4 was in development for over a year with the release date slipping multiple times. Firefox could simply not continue on it’s current path and stay relevant in the current market.
Firefox 4 is set to be released on March 22, 2011. Versions 5, 6 and 7 are also currently planned for release in 2011.