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."