How to enable firefox's (3D) hardware acceleration?

I cannot remember very well... But I think that there is a difference between what is enabled for firefox by default, in windows and in gnu/linux related to graphics performance. And because of that one should tweak something at about:config, as I said, I cannot recall very well but I still remember that I saw something related to that in this forum.

Such thing probably(?) is fixed in more recent versions of firefox but what if I am using the firefox version of Slackware 14.2 (?

I am looking for what that tweak at about:config is.