My understanding is that syncing with Firefox bookmarks is difficult because you can have sub-sub-sub-sub folders, but Android's browser doesn't have nesting like that.
My solution has been to use Xmarks on Firefox (an extension that every Firefox user should install IMHO) and then create a bookmark to the mobile "My Bookmarks" section on the Xmarks site. It isn't a perfect solution, but it is workable for me.