Trade-offs exist! How astonishing.

From https://wiki.mozilla.org/Performance/MemShrink:

"MemShrink is a project that aims to reduce Firefox's memory consumption. There are three potential benefits.

1. Speed. Firefox will be faster due to less cache pressure, less paging, and fewer/smaller GC and CC pauses. Changes that reduce memory consumption but make Firefox slower are not desirable.

2. Stability. Firefox will suffer fewer aborts/crashes due to virtual or physical memory exhaustion. The former is mostly a problem on 32-bit Windows builds with a 2GB or 4GB virtual memory limit, the latter is mostly a problem on mobile devices that lack swap space.

3. Reputation. Fewer people will complain that Firefox is a memory hog and that Mozilla ignores memory usage."

I have already found one case in Firefox where this is true. FF will load the image data in background tabs, but it won't decode it (e.g. render a JPG) until you switch to that tab. I may be nitpicking but the extra half-second or so bothers me like texture pop-in in a game. So I go to about:config and set image.mem.decodeondraw and image.mem.discardable to false.

By now I think it's well-known to every Firefox user that in the case of Firefox, the memory used is mostly leakage.