Ublock Origin is one of the most popular add ons, and I think it would be great to integrate it directly into Firefox and Firefox Mobile as a setting. Perhaps it could be part of the 'strict' setting of the tracking protection, or it could be a separate setting for blocking ads.
Microsoft Edge has a built in optional ad blocker on mobile, and I think having this in Firefox on desktop and mobile would be a great benefit, rather than relying on users knowing about it and having them download it from the add on store.