It used to be that the only way to track visitors was via web logs. No visitor can escape them. If they request a page or any content from your site, you have a log of it along with any information your server collected.
This usually meant maintaining software to analyse your logs, making sense of all that raw data, and rolling it up to meaningful trends and other higher level views.
The irony is that when I warn people who chose to use these third party vendors to track visitors that they would not see a good portion of Firefox users in their statistics, they’d respond that it doesn’t matter because most of their visitors are not using Firefox. I’d say, how do you know that. You can see where this is going. Firefox users with the noscript plug-in don’t appear in their data, so they are like stealth visitors that they never know about.
The thing is, despite the noscript plug-in being one of the most popular, it still requires a person to install it after installing Firefox. Not all Firefox users have the plug-in.