Twitch & YouTube by default. AdSlayer now runs only on Twitch and YouTube unless you choose otherwise, so it sits alongside your usual ad blocker without overlapping. Switch “Block ads on” to Every site in the popup to use it as your only blocker. Existing users stay on Every site.

Twitch

Side-by-side display ads (the banner Twitch draws next to a shrunk stream) are hidden and the stream is restored to full size.
Fewer purple “commercial break” screens when watching several streams at once: fixed a race where a stream could start before your proxy settings loaded.
A stream stuck on the purple screen for 4 seconds reloads itself (at most twice per 10 minutes, so it can’t loop).
Proxy login is now sent with the first request, so Firefox makes half as many connections to your proxy.
The popup’s proxy status reflects what’s happening now, with a specific reason (refused, timed out, wrong password) instead of a generic error.

Popups

Unwanted popups are now prevented rather than opened and closed, so there’s no flash and no broken pages. Links and buttons you actually click still open normally.

Other

Diagnostics panel in the popup to help troubleshoot a site.
Better coverage from the filter lists: support for more rule types and uBlock’s full rule set.