The perf improvements of Quantum have nothing to do with extensions, actually. Firefox removed the old extensions model because it was literally just "here's o

The perf improvements of Quantum have nothing to do with extensions, actually. Firefox removed the old extensions model because it was literally just "here's our code, hook into it however you want", where everything was a public API. This means addons broke all the time and it was effort to keep things semi-stable. With the multiprocess stuff most addons had broken anyway, except Firefox took a model where multiprocess would only turn on if you had compatible addons. But it was going to move off it; which it did in a previous release, disabling non-multiprocess compatible addons (I think). So the addons stuff is at best related to Firefox becoming faster because of multiprocess, which happened a couple releases ago. So the decision to kill the old extension API was related to the timing of the multiprocess stuff and also just because it was a long time coming and let Firefox move to a cleaner architecture. The new API is growing, though. Firefox can already support many kinds of addons that Chrome doesn't, including stuff like Tree Style Tabs or Tab Center.