Nearly six months ago, Mozilla said that Firefox 4 would be released in November and that if it wasn't ready by then, the company would skip a December release and prep for a 2011 release. Today, the browser maker confirmed that a November release isn't feasible, so a 2011 release it is.
The primary culprit for the delay? Bugs. Or, as Mike Beltzer, director of Firefox, so artfully put it in a post to the Mozilla.dev.planning Google group, "Completing this work is taking longer than initial estimates indicated as we track down regressions and sources of instability."
But even with the scaled-back feature list, Mozilla has encountered enough bugs and stability issues to force it to go to plan B. While the browser wars are still hotly contested and quicker, more frequent updates are now de rigueur, Mozilla doesn't want to take a chance by releasing a buggy browser. According to the new timeline posted on the Mozilla wiki, Firefox 4 is presently due for release in early 2011.