Changed win32 default path to use 'Program Files (x86)' instead of 'Program Files'

Best I can tell, there is no officially-supported 64-bit version of FF at the moment:

https://support.mozilla.org/en-US/questions/782154
https://developer.mozilla.org/en-US/docs/Supported_build_configurations

Unofficial? 64-bit version indicates there isn't an official one?
http://www.firefox64bit.com/

On my machine, both Aurora and FF are installed in 'Program Files (x86)', which makes sense for a 32-bit program (http://www.samlogic.net/articles/32-64-bit-windows-folder-x86-syswow64.htm) so I think this may be a better default.