The way it is appearing is that Mozilla are scared of IE especially when you consider where the product has evolved from IE6.
Additionally, it needs to be said that whilst IE9 may be lacking some HTML5 and CSS3 features that Firefox 4 supports that HTML5 standards won't be completed until 2014, and Mircosoft will have its IE product fully supporting HTML5 by then.
I'm a firefox user but I still believe that Mozilla had no reason to do this, other than that they are realizing that IE has become a decent competitor again and they can't just pump out a slightly better browser to convince the crowd to convert. They've made themselves look a little stupid especially with regards to standards which won't be completed until 2014, and you can probably expect Google Chrome team to release a Chrome Vs. FF4 comparing its standards etc.
I think Mozilla is worried that people will accept IE 9. If Microsoft's pitch takes hold in people's minds, the push for Firefox will lose steam.
And all of this "HTML5 standards won't be completed until 2014" talk is kind of a semantic game. That fallacious concept is why Ian Hickson is trying to move away from the unrealistic notion of a "totally finished standard." In practice, most of the stuff Firefox implements is already nailed down. IE is unquestionably lagging.
Additionally, it needs to be said that whilst IE9 may be lacking some HTML5 and CSS3 features that Firefox 4 supports that HTML5 standards won't be completed until 2014, and Mircosoft will have its IE product fully supporting HTML5 by then.
I'm a firefox user but I still believe that Mozilla had no reason to do this, other than that they are realizing that IE has become a decent competitor again and they can't just pump out a slightly better browser to convince the crowd to convert. They've made themselves look a little stupid especially with regards to standards which won't be completed until 2014, and you can probably expect Google Chrome team to release a Chrome Vs. FF4 comparing its standards etc.