Quote blocks empty in Firefox 50.1.0 on Ubuntu

[Hai-Etlik]

I'm seeing the quotes in messages as blank. The vertical extent seems to relate to the missing content, but doesn't change when changing the width of the viewport suggesting no word wrapping is going on. Selecting doesn't show anything in the missing area. Elements within the quote block (Links, code blocks) show the empty square "missing character" glyph for each character (suggesting a character encoding problem) disabling CSS eliminates the problem.

Showing source shows the content as present. It also gives the same missing character glyphs for the SGML DOCTYPE header while the rest of the document seems fine.

The problem seems to have manifested while updating from Ubuntu GNOME 15.04 using a non-package managed Firefox 50.0 to Ubuntu GNOME 16.10 using the distribution provided Firefox 50.1.0.

Google Chrome 55.0.2883.87 works just fine. Switching Firefox to safe mode (disabling all extensions) does not.

Downloading the page with wget and viewing in a hex editor shows everything as seemingly being in ASCII/8859-1 as declared by the HTTP headers.

Application Basics
------------------

Name: Firefox
Version: 50.1.0
Build ID: 20161209093710
User Agent: Mozilla/5.0 (X11; Ubuntu; Linux x86_64; rv:50.0) Gecko/20100101 Firefox/50.0
OS: Linux 4.8.0-30-generic
Multiprocess Windows: 0/1 (Disabled by add-ons)
Safe Mode: true

Selecting over the affected region and pasting into a text editor shows the missing content and characters showing the missing glyph correctly.