Reddit, Twitter and other social media sponsored posts and ads are not blocked; and I saw a YouTube ad for the first time in ages.

Generally, I run uBlock with all-but-regional block lists enabled.

Oh, and Brave has its own ads.

I don't like BAT and wish it could be removed like any other extension, but it's opt in?

Also, what's stopping you from installing uBlock on Brave? Considering Chrome doesn't ship with any adblocker, is it not beneficial (given your preference for adblocking) to provide an adblocker out of the box, given you still have the ability to use any extensions you would use with any other browser?

I suppose I opted in as a result of just clicking through some dialogues during install.

The only strong reason I have to keep using Firefox is to support ongoing development of an alternative to chrome's rendering engine on PC.

Is shields enabled on these sites?, Enabling ads/trackers = Aggressive. Regarding social, we have options to disable third-party twitter/facebook/linked in for addtional privacy.

I would try in private window mode, and ensure shields ads/trackers is enabled.

Not OP, but it doesn't block Twitter sponsored posts for me.

Clear cookies/cache, re-login. Check ads/trackers = Aggressive. We mostly fixed the twitter ads a while back.