troubleshooting broken youtube embeds

in the last 48 hours, every embedded youtube video just shows a white rectangle rather than the player.

embeds from every other video player site work, and youtube.com videos work. so it's just something broken with the embeds.

when i run firefox.exe it works fine, but when i run firefoxportable.exe it's broken.

i don't recall changing anything in browser that would affect this.

how can i troubleshoot this without erasing all my bookmarks/history/stored passwords/plugins and just starting over?

thanks