https://bugzilla.redhat.com/show_bug.cgi?id=1160935
--- Comment #10 from Nicolas Mailhot nicolas.mailhot@laposte.net --- (In reply to Jason Merrill from comment #5)
Well, it's about the interaction of firefox and the fontconfig setup on a standard Fedora system; it could be addressed in either place. I think the choice of Nimbus Sans was based on metric compatibility, which is less important for web fonts.
Nimbus is a valid (if old) Helvetica substitute it was designed to replace Helvetica.
If you want better substitutes for historical postscript fonts such as Helvetica, you should package TEX Gyre in Fedora (assuming spot confirms it passes legal now). TEX Gyre is the most modern floss postscript font replacement set.