(In reply to David Burns :automatedtester from comment #0)
The following tests fail in Firefox but pass in Chrome and Safari
/css/css-fonts/variations/at-font-face-descriptors.html [24 subtests]
This fails because we don't fully support the CSSFontFaceRule interface, I think. Some relevant bugs look like bug 443978, bug 1058408, bug 1466489. (Though details in them may well be out of date.)
▶ /css/css-fonts/variations/font-opentype-collections.html [2 subtests]
And opentype collection support is entirely unimplemented.