Some Test Pilot experiments are built using technologies such as Add-ons SDK that are going to become deprecated in near future. And these experiments(like Tab Center) can’t use WebExtensions for their full functionality. What will be with them after Firefox removes ability to use old technologies?
This was the position on this scenario back in January. https://github.com/bwinton/TabCenter/issues/868
It’s going to depend on the experiments. Many (all?) of the experiments we have today will probably be ended by then and could be dropped, or landed in Firefox itself. The add-ons could be converted to a webextension (for example, the sidebar API landed recently, which could make Tab Center possible as a webextension).
That said, we’re a branch of Mozilla with a mission to experiment and iterate quickly. If it makes sense to write code that isn’t a web extension, we’ll do that, and if we learn something worth building a full WebExtension API around we’ll move in that direction.