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Mozilla has released Firefox 15 for PCs, smartphones, and tablets. The most standout features are a completely silent background updater (like Chrome), significant memory footprint improvements, a built-in PDF reader, better SPDY protocol support, and a new native UI for Firefox Mobile on Android tablets.

Mozilla debuted an early version of the background updater in Firefox 12, but it still harried the user with various pop-ups. With Firefox 15, the browser now downloads and applies updates in the background, and then switches to the new version the next time you open the browser — just like Google Chrome.

The Firefox add-on memory leak has finally been fixed. For almost as long as I can remember, the only surefire way to reduce Firefox’s bloated memory footprint was to close it down. In theory, closing tabs should have the same effect — forcing add-ons to relinquish their memory allocations — but until now it hasn’t. Your mileage will vary, depending on which add-ons you use, but in general you should notice quite a big reduction.

The built-in PDF reader (pictured below), provided by PDF.js, is turned off by default but can be turned on by visiting about:config and setting pdfjs.disabled to false . After some preliminary testing, it seems about as capable as Chrome’s built-in PDF reader, but a bit slower. This is just a preliminary implementation though — it might speed up by the time final code ships in a later version.

The update to SPDY v3 brings Firefox 15 back into line with Chrome, which rolled out SPDY v3 in Chrome 19. SPDY is Google’s updated version of HTTP, and a direct competitor of Microsoft’s HTTP S&M. Both SPDY and S&M bring new and much-needed functionality to the very old HTTP spec, such as multiplexing, improved security, and modes that better support mobile devices. SPDY is an open source spec that can speed up surfing by up to 50%, but at the moment it’s only really used by Google’s web properties — still, you definitely notice the difference when using search or YouTube.

Finally, the desktop version of Firefox 15 has improved WebGL support (and a very cool playable FPS demo, if you want to appreciate what WebGL could do for browser-based games) — and the tablet version of Firefox Mobile has had its interface completely reworked (pictured above). If you don’t have WebGL enabled (if you use IE, for example), check out the video below.

For a complete list of changes, hit up the changelogs for Desktop Firefox 15 and Mobile Firefox 15.

Download Firefox 15 for Windows, Mac, or Linux. (Android link will follow soon, once the Play Store updates.)