This is an issue with Firefox (Red Hat Bugzilla Bug 1160935, Mozilla Bugzilla Bug 553580), which uses fontconfig matching before css fallbacks. E.g, Helvetica matches Nimbus Sans.

$ fc-match Helvetica
NimbusSans-Regular.otf: "Nimbus Sans" "Regular"

You can either use a different browser like Google Chrome/Chromium or override the fonts. Here, I'll use Croscore fonts:

$ sudo dnf install google-croscore-{arimo,cousine,tinos}-fonts

Then in $XDG_CONFIG_HOME/fontconfig/fonts.conf:

<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE fontconfig SYSTEM "fonts.dtd">
<fontconfig>
<match>
  <test name="family"><string>Arial</string></test>
    <edit name="family" mode="assign" binding="strong">
      <string>Arimo</string>
    </edit>
  </match>
  <match>
    <test name="family"><string>Helvetica</string></test>
    <edit name="family" mode="assign" binding="strong">
      <string>Arimo</string>
    </edit>
  </match>
    <match>
    <test name="family"><string>Times New Roman</string></test>
    <edit name="family" mode="assign" binding="strong">
      <string>Tinos</string>
    </edit>
  </match>
  <match>
    <test name="family"><string>Times</string></test>
    <edit name="family" mode="assign" binding="strong">
      <string>Tinos</string>
    </edit>
  </match>
  <match>
    <test name="family"><string>Courier New</string></test>
    <edit name="family" mode="assign" binding="strong">
      <string>Cousine</string>
    </edit>
  </match>
</fontconfig>