The first beta of Firefox 2.0 will be out this week, possibly today.

Developers on the open-source browser have been playing with an earlier alpha release of the browser since late March, but the beta release will hugely expand that test group and marks a further step toward the final release of what has been billed as a major update.

The new version is expected to include an "undo close tab" feature that lets users recover after they've accidentally shut a tab window. A spell checker designed to work with Web forms is also in the works.

The update comes as Firefox is gaining on Microsoft's Internet Explorer, and will come out around the same time as Microsoft released the final version of Explorer 7, which threatens to undo most of Firefox's hard work by offering a range of similar features.

Firefox is now used by nearly 13 percent of Net users, according to Web analytics firm OneStat.com, up 1.14 percent from May. It is most popular in Germany, where it is used for nearly 40 percent of Web visits.

Explorer remains the dominant browser, however, with nearly 86 percent market share worldwide.