Setting FirefoxPortable as default browser

Hi. I would like to set Firefox Portable as my default browser. I've found a few different references to methods for ding this, but they are either no longer available or they simply don't work.

I've also looked in at the FAQ here and searched the forum. Unless I'm overlooking something, the info available is how to set Firefox as the default but not FirefoxPortable.

Can this be done?

I'm running both Windows 8.1 and Windows 7 and would like to set things up for both.

Anyone know how to do this?

Thanks in advance!