Try to disable hardware acceleration in Firefox.

Tools > Options > Advanced > General > Browsing: "Use hardware acceleration when available" https://support.mozilla.org/kb/Troubleshooting+extensions+and+themes https://hacks.mozilla.org/2010/09/hardware-acceleration/ Try to set the gfx.content.azure.enabled pref to false or if this didn't help disable Direct2D by setting the gfx.direct2d.disabled pref to true on the about:config page and leave hardware acceleration otherwise enabled.

http://kb.mozillazine.org/about:config

I can confirm that the built-in PDF Viewer doesn't handle this PDF file properly. On Linux Evince doesn't handle this file as well (actually I do not see any text), so it looks that the file is not a standard PDF file, but uses some special coding that isn't compatible with all PDF viewers. You can try to regenerate the file with other settings.

This is the craziest thing!

First, it seems each character is rendered as though its character code were +3:

2 => 5 0 => 3 1 => 4 6 => 9

o => r u => x r => u

Next, if you search for assembly, the expected word (rendering wrong) is highlighted. So Firefox knows what the characteres are, and some kind of extremely bad font substitution appears to be occurring.

If I open the document in Adobe Acrobat XI, turn on the Edit Text & Images tool, select the contents of a text element and change the font to something other than Arial and then back to Arial, after saving, the font looks fine in Firefox's PDF viewer.

Acrobat uses Arial instead of ArialMT, which sounds very similar, but may in fact be just different enough to explain the discrepancy. (My system doesn't have ArialMT.)