The site bug wasn't "sites looping through non-existent lists if and only if they need to interact with a plugin". It was sites trying to detect whether Flash

The site bug wasn't "sites looping through non-existent lists if and only if they need to interact with a plugin". It was sites trying to detect whether Flash is installed by looping over the list, deciding it's not installed, and not trying to play the video (or the game or whatever), instead pointing the user to a "Download Flash" page. This affected enough sites that it was a serious problem for users, not a "very tiny bug". In any case, at this point Firefox supports exactly one plug-in, so all enumeration can tell you is whether Flash is installed or not. That's still one bit of data, of course, but that bit could be extracted even with the "no enumeration" patch by explicitly querying whether Flash is supported.