When your Firefox browser has more than 4 add-ons installed then if you access Get add-ons panel in Add-ons Manager Firefox shows personalized add-on recommendations with “Recommended for you”? box, these add-ons might be unique to each user. Surprised! your installed add-ons are tracked and send to Mozilla once a day to show these. Recommendations appear only when at least three add-ons are installed in Firefox browser.
Mozilla introduced new add-ons manager in Firefox 4 where “Get Add-ons” panel offers some useful information to early starters to customize their browser like “First time with Add-ons” video (you may not have noticed this till now), Up & coming add-ons and Featured themes (now personas) and links to browse them on Mozilla add-ons website.
To update the above information including add-ons metadata Firefox sends information about identifiers of add-ons you installed and startup time taken by Firefox with these add-ons installed to Mozilla. Add-ons gallery returns updated add-ons metadata information to Firefox for you to display. Mozilla uses startup information to improve Firefox performance.
If you’re concerned about your privacy and don’t want Firefox to send Add-on identifiers to Mozilla you can opt-out of Add-on recommendations easily, all you need to do is find extensions.getAddons.cache.enabled preference in about:config and turn its value to false. Leaving that preference doesn’t harm you either.