Opera Software today released version 11 of its flagship desktop browser, adding a new tab manager and support for extensions.

Opera's last major upgrade, version 10, shipped in September 2009.

The browser's most noticeable addition is "tab stacking," a new way to manage multiple tabs. In Opera 11, users can drag and drop open tabs to create a stack, or collection of tabs, on the tab bar, then preview the contents of a stack or re-open them as separate tabs.

At least one Opera rival has also made changes to help uses handle tab glut. Mozilla debuted its "Panorama" tab manager a separate screen that lets users shuffle tabs in an out of groups, last August in an early build of Firefox 4. Mozilla plans to ship the final of Firefox 4 early next year.

Opera 11 also finalised support for extensions , a feature that the Norwegian browser developer first announced in October just prior to launching an "alpha" preview. The add-ons can be downloaded from Opera's library .

The collection currently contains about 200 add-ons, far fewer than Mozilla's or Chrome's add-on galleries. Among the most popular, according to Opera, are the LastPass password manager, LastPass purchased the better-known Xmarks bookmark sync service earlier this month and a Web ad blocker called NoAds.

When Opera introduced extension support, the company's chief CTO, Hakon Wium Lie, said add-ons were "ripe for standardization," and called on other browser makers to work together.

"We'd like to work with other vendors to make sure that extensions become a true part of the Web," Lie said in October. "We should work together, as we've done with other standards, to make extensions part of that repertoire, that buffet, of standards."

Opera's former CEO repeated the call for standardisation today.

"We would like the different browser makers to work together and standardise extensions," said Jon Von Tetzchner, Opera's co-founder and former CEO, in an interview Thursday. "It's very much in line with our thinking about Web standards and makes a lot of sense."

Other modifications to the browser include a tweaked address bar that now shows the security rating of a site and a visual guide to mouse gestures, the small movements of the mouse that the browser reads as navigational commands.

Opera 11's new tab stacking feature helps manage multiple tabs.

Opera also touted version 11's reduced size about 30% smaller than its predecessor that will make it a faster download. Opera shrunk the download file by dropping external installers. "There's no separate installer," said Von Tetzchner. "Opera installs itself."

Opera remains a minor player in the browser market, accounting for just 2.3% of all desktop browsers used worldwide last month, according to U.S. metrics vendor Net Applications.

Von Tetzchner argued that Opera's share is under-counted.

"We've added 50 million active users this year, 50 million last year, so we've tripled the number of active users in the last two years," Von Tetzchner said. "Of course we'd like to grow it even faster, we're putting more resources into development, but we think [Opera's share] is more like 7%-8%."

Von Tetzchner acknowledged that the 150 million active users he cited included those of Opera Mobile and Opera Mini, two editions that run on smartphones . Opera's desktop-only user base, he said, was approximately 50 million.

In comparison, Mozilla claims to have 400 million active Firefox users, while Google recently boasted that Chrome is being used by about 120 million people.

Von Tetzchner also dismissed the idea that Google was pushing out browser upgrades faster than Opera.

"There's a lot in this release," said Von Tetzchner, talking about Opera 11. "What we'd call a dot release some others [bump up the version number] a full point. "Opera 10.5 was a major release, and we could have called it [version] 11 and no one would have complained."

Opera shipped version 10.5 last March, adding a new JavaScript engine dubbed Carakan that dramatically boosted the browser's SunSpider benchmark scores. In Computerworld's latest tests, Opera 11 was the fastest of the five top browsers in rendering JavaScript.

Google, on the other hand, has been adding a full point to the version number of Chrome about every six to eight weeks. The current "stable" build of Chrome, for instance, is version 8; Chrome 6 debuted in early September 2010.

Opera 11 can be downloaded for Windows, Mac OS X and Linux from the company's site.