Firefox enhancement/fix

I added a grunt task to start firefox in a clean environment with mailvelope being loaded. This has been part of the Makefile and can be quite useful.

I also identified another problem with firefox plugin not loading. I guess, we should have given the firefox plugin a try on the last weekend :D Sorry for the inconvenience. Everything seems to work fine. Now I can take a look into #161 (I was able to reproduce it)