iPhones and iPod touch models can't sync with Firefox bookmarks, but you can regularly push your bookmarks to them on a Windows system. The trick involves the Xmarks sync service, Internet Explorer and a tiny system tray utility.
It's not quite automatic, but the way blogger Shawn0 details his own solution for synchronising his Firefox bookmarks to his iPod touch, it's much better than a manual import/export of HTML files. And, as Shawn found out, some of the previous Firefox-to-iPhone solutions don't work with the latest builds of Firefox (3.0 and newer, specifically). It's not tricky process, just systematic, and requires, at most, a right-click of a system tray utility (intended for Internet Explorer users of Xmarks) before you next sync your iPhone.