Firefox ESR 31.2.0 on Gentoo Linux mangles ligatures "ff", "fl", "fi" and "ffl"

I am running Firefox ESR 31.2.0 on Gentoo Linux on an AMD64 computer with 8 GB RAM. Today I noticed that quite often, when Firefox encounters the text sequences "ff", "fi", "fl" and "ffl", it merges them into an Unicode ligature glyph. However, it seems like my computer is picking up the wrong glyph, and as a result I end up with weird characters all over my open pages. This happens to both the rendering engine as well as the program's UI.

This, for example, is how it looks like when I write "ff fl fi ffl" on the address bar.

Any idea of what's going on?