Recently, I have upgraded to Firefox 30 in order to profit from the security fixes. I was delighted with its much improved speed as well, but thoroughly aggravated with a number of very nasty bugs. Contrary to my previous experience, Firefox insisted on playing Flash videos instantly, of course despite me having had click-to-play etc. already enabled.

Anyway, to fix it, go to this, go to about:config and change the setting of

plugin.state.flash from the default value of '2' to '1'.

Save, and you're mostly set.

Unfortunately, I have found a number of situations where the browser insists on playing a video regardless, which I have not yet been able to configure away, although I have all obvious things configured to not auto-play.