I've been using Netscape and later Mozilla to read my e-mail since 1996. It works very well, especially the new spam filtering. I tried switching from Mozilla to Thunderbird and, for reasons that I never really understood, it had a much higher false negative rate on spam detection. More spam got through, even though it imported my old spam training data.

So, I'm now back to Mozilla. Maybe I'll switch to Thunderbird later on.