I'm posting this using Firefox 50.1.0 on Ubuntu 16.04 and the quote block shows up fine for me. How is your browser set up for displaying content [Preferences > Content] Under the advanced part of Fonts and Colours I have 'Allow sites to use their own fonts' checked.

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It would appear to me that the font that your Firefox is using can't handle italics very well.

The other thing is I had a problem with msttfcorefonts the other week and had to install version 3.6 of the installer via the Software Centre from here as per this thread, here: http://askubuntu.com/questions/54367...fonts-problems but I don't know if this would affect Firefox, unless you're forcing it to use MS TTF to display.