What do you want to turn off HTML5 for? YouTube, if so I recommend using an add-on to just change your user agent instead of finding the 12 or so required lines to fully disable HTML5 from playing.

You can set this pref to false on the about:config page.

• media.autoplay.enabled = false

You can open the about:config page via the location/address bar. You can accept the warning and click "I'll be careful" to continue.

• http://kb.mozillazine.org/about:config

Note that with this setting you need to click the play button twice (stop/start) on the Yahoo player in case the playing state shows wrongly, but no video plays.

To LostGod I am not too fond of extensions to block something : they slow Firefox and have often side effects on the site you look at (as blocking useful things).

To cor-el This has partially solved my problem : the video on the site does not play but apparently it loads : I see a black pop-up with a turning arrow on it during few seconds (shorter duration than before). No side effects on the site during my short trial. Is there a simple way to preclude loading ?

You would need an extension like Adblock Plus to actively block loading the media content.