Firefox 15 is almost there. You can grab it already if you know where to look and it should be officially rolled out soon enough. Being an incremental update, there aren't going to be any huge new features, but there's plenty to like. One thing that's not necessarily visible but which you'll most likely notice is a fix for memory leaks caused by add-ons.
Everyone knows that adding more add-ons will slow down Firefox, invariably. While this is inevitable as each new add-on eats up memory or CPU time, the bigger problem is memory leaks.
This is even worse since Mozilla has been working on plugging its own leaks and succeeding. Firefox is much slimmer than it was one year ago, which is why it turned its attention to add-ons.
Firefox 15 comes with a patch that solves the most common add-on leaks. The best part is that add-on developers don't have to do anything; the fix applies to all add-ons.
In one fell swoop, Mozilla fixed one of the last remaining big memory problems in Firefox. Granted, there were some kinks to iron out, the patch actually caused leaks in other places, but those have been taken care of.
Apart from being more frugal, Firefox 15 should be faster too, albeit in very specific circumstances, now that it supports SPDY v3, the latest draft version of the specifications. SPDY is not widely supported in the wild, SPDY v3 even less so, but it's good to see browsers pave the way for larger adoption.
Finally, Firefox 15 brings some WebGL improvements which should bolster performance. The one big new WebGL feature is texture compression which should result in a visible speed gain.
Firefox 15 also comes with several new tools for developers, as has been the case for each release for the past year or so. The latest stable version packs a couple of interesting features under the hood that aren't enabled by default, like click-to-play plugins and the native PDF reader.