Mozilla's VP of mobile, Jay Sullivan, announced plans to develop a mobile version of Firefox for Android. He stated his plan to release a beta version by sometime late this year. Development of the browser is currently being done and HTML5 was said to be supported, however he "made no promises."
According to the article from CNET, Mozilla was planning on a version for Windows Mobile and has a version for Nokia's Maemo. However, Microsoft made the choice to support code not used by Mozilla and until an NDK is released, we can expect development to be at a stand-still. Nokia's Maemo does not have the user base that Android had and will likely to go to. Good choice in my opinion.
The article from CNET goes on to say that all of Mozilla's Firefox browsers run off of the Gecko engine, so once the developers can get Gecko working on Android, the hardest work (hopefully) will be done with.
Image from Vladimir Vukićević