Comment 54 for bug 1051559

Based on the existing system requirements Firefox already has a hard minimum requirement of GTK+ 2.10 which was released in 2006 (I think), and it looks like it has been this way since Firefox 3. (see bug 418885 complaining about this, which someone should probably close out at this point) It looks like Ubuntu 6.06 LTS doesn't meet this, so 8.04 LTS looks like our current minimum Ubuntu LTS version. (please do correct me if I'm wrong here) That had gstreamer 0.10.18, which looks to have playbin2 (I think).

My guess is that if gstreamer support 0.10.18 were added, and you could successfully run Firefox under Ubuntu 8.04 LTS (and thus probably other distros of that time), then gstreamer could be turned on by default without practically increasing any minimum system requirements beyond requiring users of some KDE-based distros to install gstreamer (which is often needed in current distros anyway).