That's strange. If it's not the debug viewer bundle I'm not sure what's going
on off the top of my head.
The fix is something I've already been planning to do anyways for the next
release, which is to upgrade the underlying Blueprint.js UI toolkit, which
will have the side-effect of switching their icons from fonts to SVG meaning
this scrambled characters in place of the actual icons effect no longer
happens as we won't be using font icons once the upgrade is done.
It may be something specific to Firefox (based on your screenshot). Does it
happen in Chrome or IE as well?