With Firefox 14, well 14.0.1, comfortable in the stable channel, Firefox 15 is landing soon in Beta. It's got a solid list of new updates and features, but one in particular should prove very handy, a big fix for most of the add-on caused memory leaks in Firefox.
What this translates into is a much snappier Firefox for those that use add-ons, which is pretty much everyone.
The benefits will vary from user to user, but everyone should feel an improvement. People that use a lot of add-ons will see the biggest changes obviously. Likewise, those that run Firefox for hours, or days at a time will also surely notice.
Mozilla has been leak hunting for almost a year now. The MemShrink project was created to deal with memory usage in Firefox which, though better in Firefox 4, continued to be a problem.
In time, the biggest issues were tackled so by the start of the year, Firefox was already significantly snappier. There were still things that could be done, but the biggest impact would have been to fix add-on leaks which were causing the biggest problems by that point.
Thankfully, Mozilla was able to create a way of limiting add-on memory leak from the browser itself, rather than relying on developers to fix their creations. What this means is that you'll be seeing the improvements instantly for all add-ons that you use. Further, add-on leaks should be a much smaller problem in the future as well.
If you're interested in the technical details of the fix, Mozilla engineer and MemShrink lead Nicholas Nethercote who's responsible for the patch, has a long post
about it describing both the problem, the methods used to fix it and the beneficial effects.
You can see it for yourself of course, just grab the latest Firefox 15 Beta, as soon as it comes out, or get it now from the Aurora channel, which will soon be updated to Firefox 16.