The stable channel of Firefox 21 (Windows, Mac, Android, Linux) released today, adding a few notable new features such as an expanded social API and further Do Not Track options for customizing privacy settings, along with a new Firefox Health Report feature.
The Health Report is an additional "About protocol" link for Firefox that is available by typing "about:healthreport" into the Firefox address bar or as a drop-down selection under the the "Firefox > Help" menu.
The Health Report itself aims to provide a graphical representation your browser's performance, replete with average and detailed startup times, monthly running time for the application (in minutes), monthly number of crashes, and total number of bookmarks, add-ons and plug-ins. Clicking on the "Raw Data" link at the top of the page opens up a list of data queries and their results in chronological order (i.e. when Firefox was scanned and for what reason.) The "Raw Data" tab will show exactly what information is being sent back to Firefox (if "Data Sharing" is turned on.)
Along with providing user's information about their own browser performance, the Firefox Health Report also sends back aggregated, anonymous data to the Mozilla team to use in order to improve the browser. Users who want to turn off the data-sharing aspects of the Health Report can simply click on the big "Data Sharing" button at the top-right of the screen to turn it to "Off." Alternately, users can select "Options" from the "Firefox > Tools" menu, the select "Advanced" and "Data Choices" to turn off the Firefox Health Report completely.