i don't use it, but general guidlines for plug-ins:
if it is a 'plug-in' (ie. adobe flash') it needs to be an x64 version. 32 bit x86 versions will not work in an x64 (64 bit) firefox based browser. they are native intel machine code.
the unity version number also may be critical and you may need a version specific to win8.1.
try googling 'firefox plug-in unity player x64 win 8.1'
Edited by kronckew - 2/22/14 at 8:55pm