http://youtu.be/G2nGigQglYY
Tested on development build 1.07.112324.
Tested on stable build 1.04.111745.
Tested on all control presets.
Tested on a clean profile.
It is not the fire button, it's LMB specifically. Tested with nothing bound to LMB (Prim. Mouse Button).
Command line arguments: -nosplash -noPause -showScriptErrors -exThreads=7
I have no macros or anything like that.