If Mozilla didn't take backwards compatibility seriously then the JS and layout engines in Firefox could be a whooole lot simpler than they are now... Really,

If Mozilla didn't take backwards compatibility seriously then the JS and layout engines in Firefox could be a whooole lot simpler than they are now... Really, the point of WebExtensions is to establish a well-defined, maintainable API set that can be kept backwards-compatible going forward. Swaths of legacy extensions were already breaking with every release as internal APIs changed. The old system made the e10s (multiprocess) roll-out inordinately more painful and slower than it would have been with WebExtensions, for example.