It is unfortunate that nuance can be lost as the description of improvements moves from bugs, to technical blogs, to release notes, to the press, and finally to the reader, but I assure you that there are a number of memory-related improvements that have been made over the last year or so. These are all real, quantifiable improvements, but because modern web browsers are complex pieces of software it is not possible to fix all memory problems in a single patch.
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.
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.