Mozilla has brought the concept of running web apps in both Windows and Mac desktop. It can be done using the new technology from mozilla named prism. You must either install it as an add on in firefox or in the start menu.

Using Firefox Extension:
Download it from here.
After installing the addon, you must restart the browser and can select 'Create Application for this Website' from the 'Tools' menu. Here is a detailed video link for reference.

Using the Standalone Application:
Download it from here.
After installing you can find it in the Start menu on Windows or the Applications folder on Macintosh. Just fill out the URL of any website,put a name and you can select the option where it should appear. Here is a detailed video link for reference.