Some people had reported choppy playback and/or a high CPU load when using the 27.4.0 release of the Pale Moon browser – for instance when playing HD videos on Youtube. See these topic posts on the LQ Slackware forum. A bugfix update of Pale Moon was released a few days ago and according to the releasenotes, the new Pale Moon 27.4.1 addresses these issues. I have uploaded fresh packages for palemoon-27.4.1 to my package repository so that you can check that this is true.

Remember to install ffmpeg if you are running Slackware 14.2. A ffmpeg package was added to slackware-current but you still might want to replace that package with my enhanced build of it. And if you are using Slackware’s KDE 4 you must replace Slackware’s oxygen-gtk2 package with my updated version of oxygen-gtk2 to prevent browser crashes.
This is what Youtube reports about the media capabilities of Pale Moon 27.4 on Slackware:

If your Pale Moon browser shows that “MSE & WebM VP9” is not supported, you need to go into the browser preferences menu and in the “Content” tab, un-check the “Use MSE asynchronously” so that you can check “Enable MSE for WebM video”:

NOTE: let me know and do not bother the Pale Moon developers with any issues you encounter while using my Slackware package instead of the official binaries.