Everything autostarts/autoplays (no matter what)

[tz2026]

Youtube and soundcloud seem to be the worst offenders. This only happened with the most recent upgrade. There is a media.autoplay.enabled which I've set to "false".
Even with the page autoplay indicators set to off, it will start playing anything and everything. If Flash is blocked on Youtube, it autoplays HTML5.
It wasn't doing this last weekend. There is noting in the CSS or javascript I can find that changed to force it to start playing.

[megaman]

There is a media.autoplay.enabled which I've set to "false".

This is fixed in Palemoon 26, you can try the BETA, it's open to the public.
It also affects Soundcloud, I use it.

For YouTube, at least during the version 25, you can use programs like YouTube Center or Magic Actions for YouTube.
I find it odd that blocking FLASH reverts you to HTML5, as it doesn't do that to me.

You might be in Windows 7, and a suggestion that you shouldn't be doing is to disable Windows Media Foundation, in the about:config.

[Moonchild]

Don't disable the windows media foundation -- unless you really want to cripple html5 video playback by removing support for H.264

[tz2026]

Ah. It would help if the update available would have a beta button or some other (maybe hidden or difficult) way to access.