Because Apple won't allow any browser technology besides Webkit on iOS, Mozilla can't bring Firefox to Apple's mobile devices. But instead of ignoring the platform completely, they're working on Junior: a Webkit-based browser that eschews tabs and the standard URL bar in favour of a new UI.
That's not to say that you can't have multiple windows at the same time, but Mozilla has pushed many of the browser's functions down into a secondary level in the name of simplicity. Mozilla considers the current design of browsers to be outdated, and see their iPad project as a chance to experiment. So when you're interacting with the main window of the browser, all you see are two button overlays on the left and right edge of the screen's lower section (where your thumbs theoretically rest with a tablet in hand). The left side will allow you to navigate back, with a long press revealing added functions (forward/reload/function). The right button takes you into the lower depths of the browser, where you can switch between recently opened pages, launch bookmarks, and search or enter a new URL.
Along with the iPad browser, Mozilla is also working on a new feature for desktops called search tabs, which aims to replace the search bar with a new, left-side navigation bar that allows you to search between search engines everytime you launch a new search. They also have plans for "presence", which they vaguely describe as letting people talk and share directly from the browser. But really, the focus is the new iPad app, and its nice to see Mozilla trying to do something original. [Mozilla via The Verge]