I realize a browser and an email client are two separate entitites, but I tend to see them as a unit, like two hands working together to handle all my internet needs.
So, I haven't switched to Firefox mainly because Thunderbird is incapable of handling Groups of Contacts (or Distribution Lists). You can't import them or export them, and thus there is no way to synchronize my address book on two computers (even if they are both running Thunderbird). Since I use a lot of Groups (Lists), ... for me, this is a deal-breaker.
I have tried other solutions, including MSN, ... and was surprised that MSN can't even synchronize with the Windows Address Book used in Outlook Express. They are both Microsoft prodcuts, ... but they don't talk to each other! Pitiful.
So, I am still hanging on to my Internet Explorer and Outlook Express. Why? Because Outlook Express is the only product that can handle Groups successfully, and synchronizing two address books on two computers is amazingly easy. Simply export and import the .WAB file.
Even the supposedly more powerful Outlook can't do that! (Yes, the experts will tell you that Outlook can synchronize with other Outlook address books, including Distribution Lists, ... and I managed to accomplish that once, ... but the process is so complicated and un-intuitive that I couldn't remember how to do it just a few weeks later. So, if I can't figure it out, Outlook's synchronization is as good as uselees.)
So far, I haven't found any product that can successfully and easily synchronize Groups (Distribution Lists) like Outlook Express does. As long as that is the case, I will stay with the OE / IE package.
Oh, yeah, ... I haven't really found a compelling reason to switch, ... as much as I would like to. Sure, there may be some vulnerabilities in IE, ... but I figure that Norton's firewall and Anti-virus software keeps me reasonably safe. So far, I haven't had any problems.