When opening twitch.tv clips I find I am repeatedly prompted by the block autoplay doorhanger unless I grant persistent permission. This is of particular issue when watching clips in private browsing, where they will stall several times while I'm reprompted. For example, watching https://clips.twitch.tv/LachrymoseDeterminedDogeCclamChamp in private browsing mode resulted in 3 prompts (each with a stall in playback) before I was able to watch the clip through.

Possibly related to Bug 1477273?

In normal browsing, showing prompt multiple times was fixed by bug1477273. In private browsing, bug1478869 would add a new checkbox in order to keep the life length of permission as same as session's life length.