It was added back in
5147d5c69f
for unclear reasons (and the lack of bug number doesn't help), and hasn't been
used, as far as I can see in the gecko-dev history, other than in bug 206029,
which is the only use currently in the tree.
Bug 206029 was working around the Flash player installer modifying Firefox's
prefs file and not dealing with it properly or something depending on the line
endings. 11 years later, all prefs files except channel-prefs.js are in
omni.ja, so obviously, bug 206029 doesn't actually apply anymore.
So, let's simplify it all and get rid of this.
7f68f92edb