plugin location /usr/lib64/mozilla/plugins not used in firefox 47.0.1, works in 38.8.0

I'm running slackware 14.1. I have /usr/lib64/mozilla/plugins/libflashplayer.so (Installed from alienbob's repo) If I run firefox 38.8.0, it gets loaded $ strace -e open /usr/bin/firefox 2>&1 | grep libflashplayer.so open("/usr/lib64/mozilla/plugins/libflashplayer.so", O_RDONLY|O_CLOEXEC) = 48 But if I run firefox 47.0.1 it is not loaded. $ strace -e open /usr/local/bin/firefox 2>&1 | grep libflashplayer.so However, if I set MOZ_PLUGIN_PATH=/usr/lib64/mozilla/plugins, it works. $ MOZ_PLUGIN_PATH=/usr/lib64/mozilla/plugins strace -e open /usr/local/lib/firefox 2>&1 | grep libflashplayer.so open("/usr/lib64/mozilla/plugins/libflashplayer.so", O_RDONLY|O_CLOEXEC) = 44 38.8.0 is from the initial slackware install, and 47.0.1 is manually downloaded from mozilla.org, installed in /opt and symlinked to /usr/local/bin. I've attached the "troubleshooting information" from 47.0.1. How do I go about debugging this? Is this actually an issue, or is it just expected that I export MOZ_PLUGIN_PATH in my dotfiles?