Story

One of the nuisances of Firefox has been incompatible extensions across browser versions. Upgrade right away, and you might not find your extensions available for the new version. Stay with an older version (like perhaps the ESR) and you don't get upgraded versions of the extension, which is a problem when sites like YouTube change and break things...

If this move means developers don't need to make changes to their extensions for compatibility with successive versions of Firefox all the time, it could have positive effects on the ecosystem. But of-course that's only after the initial, sudden drop-off this is sure to cause.

Like everyone else, I also hate how Firefox is turning into Chrome, but that's from a UI and performance (on lower-end systems) perspective. They can turn the back-end into Chrome/WebKit for all I care, as long as the performance improves, the old Navigator UI hangs around, and the extensions keep working.