Glad to know you would like to make Firefox as default browser. for doing that do following instructions. At the top of the Firefox window, click on the "Firefox" button and then select "Options" Then, Select the Advanced panel, click the General tab, and then click Make Firefox the default browser. The Set Default Programs window will open.

For more instruction, please see this article. Make Firefox your default browser

Regards

Hello,

See: http://www.askvg.com/how-to-use-mozilla-firefox-metro-version-in-windows-8-fix-metro-app-launching-desktop-version/

In that tutorial he uses the Nightly but the process to make Firefox as default is the same

You can use the latest version of Firefox in Windows 8.1. You can download it from here.Download Firefox. As per Microsoft Compatibility Center mentioned that Firefox 27 works fine with Windows 8.1, I think there should not be problem in using it. You can find it here Widows Compatibility Center

Let us know if you have any difficulties in using Firefox in windows 8.1 Regards Safwan

Thanks to both of you for thr quick and helpful responses>