Years ago, Mozilla introduced its mobile equivalent of Firefox, then-called Minimo. Minimo unfortunately largely died of boredom within Mozilla. In early 2008, however, Mozilla resurrected Minimo as Fennec, and the heavens rejoiced (though even the heavens couldn't get it installed on [Name your mobile device of choice]).
As recently announced by Mozilla, however, Fennec just hit its second alpha release, with the option to download and install the mobile browser on Mac, Linux, and Windows desktops for testing purposes. (If you want to install it on your mobile device, you're going to need to have a Nokia N810 device on which to install it.)
Alpha 2 has made significant improvements to Fennec's performance (e.g., Faster panning and zooming plus improved responsiveness while pages are loading) and ease-of-use (e.g., Bookmarks, tabbed browsing with thumbnails, etc.)