Remember the old joke about how the Java slogan “write once, run everywhere” was more like “write once, debug everywhere”? Good one. Actually, many people say this about the Web, too. In fact, it would be more accurate in our case to state it as “write once, debug everywhere, each time with different tools”. But does it have to be that way?

A new Firefox feature called Valence allows you to debug your web app in any target environment: Firefox, Chrome, iOS, Android, you name it. Does it actually work? How does it work? Can we expect more from it? How can other browsers use it? In this talk we will find the answers to these questions and discuss the accuracy of the tentative slogan “write once, debug everywhere, from Firefox” (or lack thereof).