The all-important user interface

Like Safari, mobile Firefox will be able to work with a touchscreen but also will be available with a non-touch user interface. "We're spending a lot of time and resources on the user experience. This is really key," says Christian Sejersen, Mozilla's director of engineering.

Sejersen identifies several vital elements in optimizing that experience on a mobile device: devote as much of the screen's real estate as possible to the actual browsing experience, eliminating such things as onscreen buttons; make the interface very intuitive, so it's easy for the user to discover and use features; finally, make sure the interface doesn't hinder what you're trying to do.

As an example of his last point, Sejersen says Safari on the iPhone (which he calls a "great mobile browser") displays multiple browser windows as tabs. "If you zoom out to see multiple windows, you see a blank page: to reduce memory usage, it's thrown away," he says. "You [then] have to scroll between them to find which one you want. That takes a lot of time."

By contrast, a prototype of mobile Firefox lets the user drag the open Web page to one side, to reveal the additional pages that are open, a collection of thumbnail images: The user simply taps on the one he wants, and it fills the screen.

A recent "concept video" by Aza Raskin, head of user experience for Mozilla, demonstrated what he carefully calls a "possible direction" for the mobile browser's user interface.

The opening screen shows a big "plus" (+) button on the left, and bookmarks to the right. Click on the + button to open a tab or a new page. Click on a bookmark, and the browser zooms to the page. Scroll the page by dragging and by "flicking."

The standard browser controls, such as back and forward, are located to the left of the Web page you're viewing, as if they were waiting in the wings off-stage. To see them, you gently drag the page to one side, in effect pulling them onto the screen. The URL bar fades into prominence at the same time. This means that until you want a control-button function, the screen is completely filled with just the Web-page content.

The concept video shows a set of clickable actions at the bottom, actions that likewise are accessed by dragging the page out of the way. These actions include such things as "Digg this page."

Pan the page in any direction, and you see a big white arrow; release it, and you zoom out of the page. Or you can abruptly "throw" the page with a finger gesture to one side, and zoom out.

The mobile browser also will make use of Mozilla's Project Weave, introduced at the end of 2007 for the Firefox 3.0. A browser extension, Weave lets users save data, such as personal browsing information, to a Mozilla server and access it from multiple machines. It's a way to let users share bookmarks and collaborate, and to synchronize between the desktop and mobile versions of the browser. "You'll just walk away from the desktop browser and pick up where you left off, on your phone [browser]," Sullivan says.

But it's not all about the user interface. Mozilla designers earlier this month fired up their Talus test environment for mobile Firefox. Talus runs numerous page load tests, and measures how long it takes, emulating a mobile network for the browser. The results will be used to further revise and tune the mobile browser for optimal performance over cellular networks.