If the Firefox browser were a car, it would be in the garage right now being souped up by an anxious group of gearheads.

When Firefox 3.0 is released later this year, the open-source browser is likely to contain a host of new features, including offline support for Web applications and new bookmark and search features. Mozilla released the second alpha version of Firefox 3.0 earlier this month.

While the final feature set hasn't been determined, Firefox 3.0 will also contain elements for its 4.0 release and beyond, said Mike Schroepfer, vice president for engineering for Mozilla, during a stop in London on Tuesday. The browser is due out in the second half of the year.

"What we're trying to do with all of these things is lay the foundation," Schroepfer said.

Perhaps most exciting could be Firefox's ability to support writing an e-mail in, for example, Gmail while offline, with the data sent later when a user is connected to the Internet again. Ultimately, Mozilla engineers are aiming for an integration between the browser and Web-based services that is as smooth-running as a desktop application, Schroepfer said.

So far, engineers have made Firefox work with Zimbra, an open-source e-mail, messaging, and VoIP application. With a bit of code from Google and Microsoft, it would be possible to integrate with Gmail and Hotmail and other e-mail services.

To do offline support, engineers have overcome the hurdle of how to store data locally on the computer, Schroepfer said. The feature will make it into Firefox 3.0, although the user interface is still under development, he said.

Other changes could come to the bookmarks and history features, which have seen relatively little innovation, he said. Mozilla would like to create a function where bookmarks could be automatically sorted based on popularity and frequency rather than the static presentation now.

Firefox 3.0 will also have a small, embedded database -- SQL Lite -- that will eventually be used for full-text indexing of the browser's "history." Users could search for images and text and see the cached page. The feature, however, may not make it into the 3.0 release, he said.

"The advantage of the database is that we can search your cache," Schroepfer said.

Most importantly, Firefox has to be fast and standards-compliant, he said. Some users have complained about Firefox sucking up processing power because of add-ons or extensions, a popular aspect of Firefox where small programs can be downloaded into the browser to add new functions.

But extensions sometimes tax system resources, in part because it's often part-time hobbyists doing the coding, Schroepfer said.

Mozilla will soon set up a shared library of tested code that extension writers can download and use, Schroepfer said. Mozilla also relaunched its extension site, cutting back on the number of extensions listed so first-time users don't overload their browsers and dampen their experience.

In a few weeks, discussion forums will also be set up for developers to exchange feedback as code writers sometimes don't know of the problems, he said.

Firefox 4.0 will support the JavaScript 2 language, now under development in part by Mozilla's CTO Brendan Eich and the ECMA International standards body, Schroepfer said. The idea behind the JavaScript revamp is to make high-performance Web applications easier to write and assemble for people with less coding expertise, he said.

"It's less about making it possible and more about making it easy," Schroepfer said.