This latest fix is for misbehaving addons. Other, previous fixes, have solved a lot of problems with FF itself. For me, it's less memory hungry than Chrome.
If you look at various comparisons of browser memory usage (which are admittedly somewhat tricky to do), Firefox has, in the last 6 months or so, consistently been at or near the top of the ratings. This was not always the case, and is the result of cumulative improvement over many releases.
To the topic immediately at hand, what Firefox 15 does for memory usage is that it eliminates the single most common source of unbounded memory leaks in addons. The problem was that addons would accidentally leave behind a reference to pages, keeping them alive after they were closed, using up gobs of memory. Now, if addons try to do that, the link is killed, preventing the leak. Arguably, this is a bug in an addon and not the browser, but this has affected literally dozens of addons used by millions of people, so it called for a hardening of the browser itself to prevent this error.
What is true is that multiple releases addressed separate, significant problems. It's important to remember that there is no 100% solution to a complex problem like memory usage in a large program that runs arbitrary code (the web).
A lot of useless stuff yes, the useful stuff? Not really. Good luck building something as wide-ranging as Firebug through Chrome's API.
The important thing is you have choices and can pick the browser that makes the tradeoff you prefer.
Are you an extension developer? Have you ported an extension from Firefox to Chrome? Opinion is one thing; please don't misrepresent facts.
If you look at the release notes, we claimed improvements in add-on memory consumption for Firefox 15, and general improvements for the browser itself in Firefox 7. (And also Firefox 3, I think, but that's ages ago predates my time working on the project.)
I know this because I wrote the accompanying blog posts for those two releases (https://blog.mozilla.org/nnethercote/2012/07/19/firefox-15-p... and https://blog.mozilla.org/nnethercote/2011/08/09/firefox-7-is...) and liaised with marketing about the release notes.
I really want to understand where this misconception comes from. I think repeated reporting in the tech press of the same improvement as it moves from Nightly to Aurora to Beta to release might be the cause.
(Forgive me for replying twice to your comment; this comment prompted me to go do some research and write that post.)