The browser wars are back!

FORTUNE — The long, cold war over the Internet browser is heating up again.

Recent statistics from StatCounter indicate that Microsoft’s (MSFT) Internet Explorer may no longer have quite the hold on the market. According to the web analytics company, which relies largely on page views for its methodology, Google (GOOG) Chrome seized the top spot with 32.4% of the overall browser market share for the month of May, compared with IE’s 32.1%. Rounding out the top five were Mozilla’s Firefox browser (25.6%), Safari (7%), and Opera (1.8%).

May marks the first time that Internet Explorer has lost its lead since at least 2008, when StatCounter began tracking data, although the swap is not surprising given the browser’s steady decline: its market share has fallen nearly 11% since just May of last year. Those numbers have come under some dispute by other web analytics firms like Net Applications, which tracks browser market share based on unique visitors and factors in different use cases in varying countries. By their measurements, Chrome’s 20.2% share edged out Firefox for second place.

Even more important for companies like Google, Microsoft, Apple (AAPL), and Yahoo (YHOO) however, is mobile.

“It’s about data and more control of the customer,” says Greg Sterling, an analyst with San Francisco-based Opus Research. That’s likely why a company like Yahoo decided to make a big play with its buzzed-about new product, Axis. A series of mobile browsing apps and desktop plug-ins, Axis takes a radical approach by ditching the conventional results page in lieu of a thumbnail-focused interface that lets users quickly jump from their search to the actual page. Somewhat similar to Chrome, Axis also syncs data like say, browsing history and bookmarks across various devices. The product is an opportunity to get a new search experience in front of consumers, but also potentially opens up new ways for Yahoo to make money.

Rumors also continue to swirl around the possibility of Facebook (FB) acquiring Opera (OPESY), the Norwegian software company started in 1995 by Jon Stephenson von Tetzchner and Geir Ivarsøy. Originally intended as a research project at the Norweigian telecomm company Telenor (TELNY), Tetzchner and Ivarsøy got the rights to their software, formed Opera as a separate company, and began developing browser software for computers and phones.

To be sure, Opera’s 1.72% global market share is tiny. But the company reported nearly $47 million for the first quarter of 2012 thanks in large part to licensing agreements with companies like Nintendo (NTDOY), which offers it on its Wii console, as well as partnerships with companies over its in-browser search and shopping bars. Over 168 million people were using its Opera Mini app as of March, a 64% increase year-over-year, which indicates significant growth. The company’s shares initially rose 26% on speculation of a Facebook purchase.

Opera’s small market share may not matter if it gets assimilated by Facebook, with its 901 million monthly active members, 500 million of whom are also mobile. Fueling further mobile growth for Opera could just be a matter of devising a solution where Facebook mobile users are somehow exposed to Opera, which might include integration of the browser into the mobile app.

What will matter more for Facebook beyond Opera’s number is strengthening its mobile strategy. As Fortune reported in May when the social network opted to buy Instagram for a deal then-valued at $1 billion, Facebook’s has lagged behind the competition. Where mobile browsing offerings from Apple, Mozilla and Yahoo often hum, Facebook’s apps have stumbled, with buggy apps that are at best functional but also occasionally unusable. According to Mary Meeker, a partner with Kleiner Perkins Caufield & Byers, there are already 953 million global smartphone subscriptions, but that number pales in comparison to the world’s 6.1 billion overall mobile phone subscriptions, meaning the upside — and there is a huge one — for a company like Facebook is one it can’t ignore.

Facebook also currently lacks a strong mobile advertising platform beyond the “sponsored stories” that began showing up in mobile news feeds. With a browser like Opera under its belt, it could sell the default search engine used, assuming of course Facebook decides it doesn’t want to dominate search, also.

It’s a tactic that has largely worked out well for Mozilla, at least until recently. Its Firefox browser became popular for its open developing environment and heavy customization options and relies on lucrative search deals to generate most of its revenues. In 2010, 98% of its $121 million in revenues came from search. But Mozilla has been late to mobile. Although it has an Android app, it doesn’t have a dedicated iPhone browsing app. (Its Firefox Home app for Apple devices is less a full-fledged browser and more a way for users to access desktop history and bookmarks on-the-go.) With less than 1% of the mobile browsing market, it’s an issue the company is tackling by hiring more talent and making mobile, particularly Android, a priority.

Which is to say, the browser wars aren’t just heating up yet again. In some ways, at least where mobile is concerned, they’re just beginning.

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