So from reading the website, I don't understand what this framework does for me. I can get "cross device compatability" by using plain old javascript and html myself.
Joshfire share the same will of easing Web Application development by introducing template engines, MVC coding style, magic UI binding to data and so on. However, until now, each framework had its own supported platforms: mobile webkit for some, recent desktop browsers for the others. Joshfire targets platforms ranging from IE6 to mobile devices, connected televisions and even headless devices."
you don't "need" it but it will save you time by already packing all glue and standard code for the supported platforms (SDKs, specific video components on TVs, scrolling on mobiles, code for handling keyboard/remote keystrokes, ...)
"What differs from other JavaScript Frameworks ?
Joshfire share the same will of easing Web Application development by introducing template engines, MVC coding style, magic UI binding to data and so on. However, until now, each framework had its own supported platforms: mobile webkit for some, recent desktop browsers for the others. Joshfire targets platforms ranging from IE6 to mobile devices, connected televisions and even headless devices."