Phabricator messes up tab spacing in patches. It should be very easy to replace tabs with spaces and require the use of only monospace fonts.

FF says it uses DejaVu Sans system font. This is <pre></pre> area, so only monospaced font should be used.
As I understand from http://en.wikipedia.org/wiki/DejaVu_fonts , only some DejaVu fonts are monospaced (ex. DejaVu Sans Mono), and one used by FF isn't.