The move to WebExtensions was painful, but it also made it possible to easily port Chrome extensions to Firefox, which was a great boost for the extension ecosystem, as well as being the thing that actually made mobile extensions possible.

I do agree they should've made the transition period longer though. There were like two years in between where some of the big Chrome extensions hadn't been ported yet, but their original Firefox counterparts were already killed. That probably made a few users move ti Chrome, but that was already during the great Chrome migration, so I can't imagine this made a huge difference.