The current version 6.42 build 42 is so so.
It loads in youtube videos MOST of the time but not sometimes.
Sometimes it loads but only shows 360p available for download.
I keep having to reload and reload and close and reopen pages in hopes it will load or load properly with all the video versions to download. It eventually works properly. Not too great tho. Could be better.

PS: I AM on firefox portable (141.0)