Users can disable the data reporting from the FHR display, which can be called up by selecting "Firefox Health Report" from the browser's Help menu.
More information about FHR has been published on Mozilla's support site.
Two of the additions -- in-browser access to social media and FHR -- are examples of unique features Mozilla has added to Firefox as it tries to stay in second place. According to measurements taken by Net Applications, Firefox was used by 20.3% of the world's online users during April, more than enough to trump third-place Chrome's 16.4% but still less than half the 55.8% share of Microsoft's Internet Explorer.
After slumping slightly in the second half of 2012, Firefox has regained those losses and returned to a share about equal to what it had a year ago.
Firefox for Android was also updated Tuesday with support for a pair of open-source fonts Mozilla claimed make text easier to read on smaller screens, and other under-the-hood changes to improve the browser's compatibility with HTML5, the next-generation website design language.
Along with the new features and enhancements, Firefox 21 also patched 15 vulnerabilities, nine rated critical, Mozilla's highest threat ranking, five tagged as "high," and one labeled "moderate."
One of the critical bugs was reported by Nils, a German researcher who goes only by his first name. Nils is a notable vulnerability researcher, one of a two-man team who won $100,000 in early March for hacking Google's Chrome at the Pwn2Own contest.
Also patched were two vulnerabilities in the Mozilla Maintenance Service on Windows, which powers the browser's silent updates. A different bug in the service was quashed by Firefox 20 when it debuted in early April.
Security researcher Abhishek Arya, a Google engineer better known as "Inferno," was credited with reporting six memory corruption flaws. Inferno specifically, and Google's security team generally, have reported scores of vulnerabilities to Mozilla based on their "fuzzer" stress testing.
Windows, Mac and Linux editions of Firefox 21 can be downloaded manually from Mozilla's site; already installed copies will upgrade automatically. Users of Firefox for Android can retrieve the update from the Google Play store.
The next version of Firefox is scheduled to ship June 25.
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