I was unable to get Duo Trusted Endpoints to work with Firefox out of the box. I diagnosed the problem, at least partially, and submitted a bug to Firefox with a patch that lets me work around it:
They are looking for some more detailed analysis before including that patch, and it’s difficult for me to do that. Can anyone from Duo jump in there and help out? It seems like Duo is doing something weird that is triggering LOAD_ANONYMOUS in Firefox.