[xmonad] How to diagnose programs not launching?

[Joseph Garvin]

I have a key set to runOrRaise "firefox3". Recently, it stopped
working and I can't figure out why. Running firefox3 manually from the
shell works fine, just not the shortcut. Is there a way to see what
errors xmonad is receiving when it tries to run the command? I already
tried looking at .xmonad/xmonad.errors, it just says:

on the commandline:
    Warning: -no-recomp is deprecated: Use -fforce-recomp instead

Which I suspect is related to xmonad rather than my firefox call.