It was reported against 0.4.15-dev-628-gf363c27.
I prefer to have a ticket for that as well, because it does not just happen during initial Gecko installation as pointed out by Kyle_Katarn, but also in some audio or gameplay scenarios and may massively impact the performance then.
err:(dll/opengl/opengl32/wgl.c:747) Error: hdc is not a DC handle!
It disturbed me as well in the past, but I never had the balls to mute it, because I was not sure, whether it would point towards some more serious bug otherwhere or not.
The code went hot in SVN r60281 when opengl32,dll was replaced by opengl32_new.dll.
The JIRA ticket serves for me to have a reference when backporting it into 0.4.14.