> at least in my experience, made extension developers better at building extensions that don't break because I have upgraded from version 15 to 16 like we had back in the Firefox v2/3/4 years.
This is more caused by Firefox stopping to break the compatibility all the time. The plugin API in Firefox used to be just the internals, exposed to JS, so everytime something changed, it broke extensions. These days, they stopped doing that quite as often plus they published the Jetpack SDK which promises a stable API at the cost of less possibilities to change the browsers behaviour.
This is more caused by Firefox stopping to break the compatibility all the time. The plugin API in Firefox used to be just the internals, exposed to JS, so everytime something changed, it broke extensions. These days, they stopped doing that quite as often plus they published the Jetpack SDK which promises a stable API at the cost of less possibilities to change the browsers behaviour.