Dave!!!
Try this build, https://bitbucket.org/dryeo/dry-comm-esr31/downloads/firefox-45.9.0.en-US.os2.zip.
Still has the minimized cpu usage but does play youtube videos and closes on exit.
Same code excepting some optimizations and compiler.
Amazing...seriously, first time - in I can't remember how long - that FF actually plays native VP9 YouTube videos...so that's pretty awesome. When the h264ify add-on is turned ON the stuttering video continues, just as it did with the GA1.1, so there must be something else to it beyond the compiler stuff.
OK, so here are two requests, not sure if you can/are-willing to do this but, can you:
1) release that last test build with '-Os -march=pentium4' instead of '-Os -march=i686'? This would give me the closest to apples-to-apples comparison, so maybe the move to GCC 5.1.0 is all the difference that we need?
2) release that last test build with '-march=amdfam10 -Ofast -msse2' instead of '-Os -march=i686'? This would give me the AMD Phenom II CPU optimizations I wanted to try out (yeah, I know it violates some of the strict standards compliance, and maybe is incompatible with the other options I see in 'about:buildconfig', but if possile, please give it a try)
Thanks for putting out this alt builds!!!