At least one site, GitHub, has certainly deployed changes as a result (and also pointed out that the author of Firesheep gave absolutely zero warning to the sites it targeted, which unnecessarily left a great many users vulnerable on sites like GitHub that would have otherwise been able to close the hole before Firesheep made it public information).
unnecessarily left a great many users vulnerable on sites Except that Firesheep didn't expose any new security problem. "Hey everyone, in a few weeks I'm going to make it easier for people to exploit long-known security problems that you may have been happily ignoring" is different to "Hey everyone, I found a brand new vulnerability and I want to let you fix it before I publish it".
It doesn't matter if it's not new information or if the companies knew about it or not. It's something that should be done to avoid screwing people over. Users don't always know about this stuff. Before publishing a tool that makes it so easy to exploit users info it's a common practice to inform the companies related ahead of time in case they are motivated to fix this (such as github did) and perhaps the users so they can have heard about it before everyone has it and they are still logging in like this.