Comment 61 for bug 233901

> It works as advertised (for Linux AND Windows.) I found the problem I was
> having was that I had left some comments in. Apparently comments starting
> with # will cause Mozilla to abandon the rest of the config file.

A preference file is a JavaScript file. If there is a syntax error, than the
application will abort reading it and none of the preferences defined take
effect. With exception of user.js, however, the first line is never read. The
first line is usually non-parseable and begins with # (not a valid js syntax).
This prevents inadvertant loading of preference files from the outside.

for more info, see bug 178685