However, we do have concrete plans for supporting adblocking as a built-in feature in the near future (https://github.com/mozilla-mobile/fenix/issues/96), which should make Preview more palatable to folks who rely on add-ons for that, even if only as a stopgap.
More fundamentally, when considering major features like add-on support, we want to take the time to ensure that we're getting our design right at all relevant levels: Firefox Preview, the underlying GeckoView library, and the Gecko engine itself. And though development on Preview moves quickly, there's still a lot of design and development work yet to be done.