In a security warning, Firefox director of security assurance Michael Coates, said that "the vulnerability could allow a malicious site to potentially determine which websites users have visited and have access to the URL or URL parameters." But he said that Mozilla had seen no evidence that the bug was being exploited by in-the-wild attacks.
Even so, "Firefox 16 has been temporarily removed from the current installer page and users will automatically be upgraded to the new version as soon as it becomes available," said Coates. Furthermore, he noted that "as a precaution, users can downgrade to version 15.0.1," though he said that any current version 16 installations would be automatically updated after the patch gets issued.
Mozilla plans to issue a patched version of Firefox 16 Thursday.
[ Security threats are growing. Read Web API Allows Phishing Attack. ]
In the interim, however, is downgrading to Firefox 15.1 wise? According to vulnerability information provider Secunia, Firefox 15.x contains numerous "highly critical" vulnerabilities that could allow an attacker to remotely bypass the browser's security controls, execute a cross-site scripting attack, spoof websites, crash the browser, or access a user's system. All told, it said there are 23 issues, which remain--as yet--unpatched in version 15, though Mozilla reported fixing 14 known vulnerabilities in version 16.
According to market watcher Net Applications, as of September 2012, Firefox controlled 20% of the desktop browser market, putting it behind Internet Explorer (54%), but ahead of other rivals, including Chrome (19%), Safari (5%), and Opera (2%).
The privacy tool, which is a browser add-on, forces an encrypted HTTPS connection on any website that offers users a choice between HTTP and HTTPS. "Many sites on the Web offer some limited support for encryption over HTTPS, but make it difficult to use," according to the HTTPS Everywhere download page. "For instance, they may default to unencrypted HTTP, or fill encrypted pages with links that go back to the unencrypted site. The HTTPS Everywhere extension fixes these problems by using a clever technology to rewrite requests to these sites to HTTPS."
Despite the branding, however, the tool won't work everywhere, although it does now work with 1,500 sites--or about twice as many as with the previous version--and gets used by about 2.5 million people. "Our current estimate is that HTTPS Everywhere 3 should encrypt at least a hundred billion page views in the next year, and trillions of individual HTTP requests," said EFF technology projects director Peter Eckersley in a blog post.
So far, stable versions of HTTPS Everywhere have only been available for Firefox. But the EFF and Tor, together with a group of volunteers, are developing a version for Chrome and Chromium, though it's not yet ready for public release.