Another six weeks, another Firefox version. This time around it's Firefox 17 that has hit the stable channel. As has become the norm since Mozilla implemented its rapid release schedule in 2011, this version introduces a couple of larger features alongside a handful of small ones. Let's jump right in.

Firefox 17's banner feature is a new Social API which is sort of similar to the search API that allows different services to use Firefox's search box. The only service that currently plugs into the social API is Facebook Messenger, but anyone can leverage the API. Navigating to the Facebook for Firefox page while running Firefox 17 and clicking the Turn On button will enable a persistent sidebar in your Firefox window that shows you your Facebook chat list as well as your outstanding notifications. Pop-up notifications will also appear in the lower-right corner of your screen.

Also new is what Mozilla is calling "Click-to-play plugins." If the user is using outdated versions of plugins like Flash, the browser will no longer play content that uses those plugins by default. Users must manually click the content to get it to play and are also offered an update link that will attempt to find and install a newer version of the affected plugin if it exists. A plugin icon next to Firefox's "Awesome Bar" will also show the user more information about the disabled plugins in question.

This is also the first new version of Firefox to completely drop support for OS X 10.5—Chrome dropped Leopard support in version 21 a few weeks ago, and Safari's last Leopard-supporting update came way back in July of 2011 with version 5.0.6. Opera 12, then, is the last major browser that continues to support the aging OS, and even in that case PowerPC support was dropped long ago. Leopard holdouts may want to rethink their decision not to upgrade.

Finally, Firefox 17 ushers in Mozilla's second Extended Support Release (ESR), which will fully replace version 10 when it is retired in February of 2013. As we discussed early this year, the ESR was Mozilla's attempt to placate IT administrators upset about Firefox's then-newish rapid-release schedule. The ESR version of Firefox receives new security patches as they're released but doesn't get any of the new features present in later Firefox versions until the next major version. The ESR was a nice thought, though our browser stats don't appear to show a particularly high number of Firefox users sticking with version 10 of the browser—it's more likely that the ESR appeals to (and will continue to appeal to) a small but vocal minority.

And finally, as usual, there's a laundry list of smaller fixes and changes aimed at addressing smaller complaints—some Awesome Bar icons are slightly larger, some 20 "performance improvements" have been implemented, and there have also been some improvements to developer features like the Web Console, Debugger, and Developer Toolbar. For the full list of changes, you can check the What's New page for Firefox 17 here.