font-family tooltip
If you hover over a font-family property in the Rules view, you will get a tooltip with a sample of that font:
Fonts view
In Firefox 47 only, the Fonts view is disabled by default. If you want to see the Fonts view in Firefox 47, visit about:config, find the preference devtools.fontinspector.enabled, and set it to true.
Before and after Firefox 47, the Fonts view is enabled by default.
The Fonts view shows all the fonts in use by the selected element. Note that it shows the font used on your system, which may be different from the fonts specified in the CSS:
The text in the font view is "Abc" by default, and starting from Firefox 41 the preview text can be freely edited.