Nick,
I have been a firefox fanboy since 0.8. I switched initially because of the fact it didn't use ActiveX, but have stayed because it is a great overall browser. I no longer hold onto the "FF is more secure than IE" as each have their security holes. The number one thing that keeps me with FF is the fact it complies to most internet standards, unlike IE. At the present point in time, IE7 scores a puny 14/100 on Acid3 while the current FF scores 71/100 and the next beta release scores 93/100.
So far, you are the only person to say that Firefox secretly collects your browsing habits and uses them. At the moment firefox monitors how many browsers check for automatic updates as a way to monitor how many people use firefox and have a project in the pipelines to monitor browser usage, but it is completely open to the public and it is elective as well, if you don't want it monitored you can so choose. Also, firefox has recently stopped collecting unique identifiers on crash reports to increase end user privacy. Source: http://blogs.zdnet.com/open-source/?p=3274
I don't blindly follow what everyone on the internet says, I like to track down my own info. As yet I have not found a single reliable source that agrees with you in saying that firefox collects this information and returns it home. If you can provide some proof to your statements I will listen, but at the moment I'm marking it down as a MS fanboy slandering the competition.