If firefox does not correctly assume that your files are UTF-8 encoded, a possible solution (besides using a Byte Order Marker in your file, which is annoying for several reasons) is the following:
go to about:config and set the intl.charset.fallback.utf8_for_file variable to true. That worked for me.

This is related to the following bug report:
https://bugzilla.mozilla.org/show_bug.cgi?id=1071816