On Android < 3.0 or on Firefox v8 or earlier I think you still have to use the clunky, not-so-intuitive method. You swipe the screen to the left and it will reveal a sidebar on the right side of the screen which has the bookmark star. Press that and it bookmarks the page. How they expected to you to find this on your own I will never know.
On Honeycomb devices with Firefox 9+ (currently beta) you just need to press the little star icon on the far right side of the address bar. It pretty much works just like the desktop version.
Annotated screenshot for the right sidebar: