On April 18th 2016 I noticed that all the sites using Google Fonts display them in the standard Arial/Times New Roman combination instead of those that I've chosen to import from Google Fonts using the @font-face direction.
What's Wrong?
It turns out that the problem with Google Fonts not showing in Firefox is because I also have NoScript add-on installed, that blocks these fonts even after going to settings and disabling of blocking of font-face command.
How to Solve This Issue?
Install the latest development version, at the time of writing it's version 2.9.0.6rc1 of NoScript, that's available at developers site.