I love the way that Firefox and Thunderbird can be extended by extensions. But I've been somewhat concerned about the reliability and stability of Greasemonkey scripts. There were a couple of features that I wanted to try (more on that later) so I broke down and installed it on my laptop.
Wow. I've been very pleased with how well the scripts worked. I'm still cautious about installing them, since they're not looked over anywhere near as much as extensions on the official Mozilla site are. But the Userscripts site lets you look over info from the author and comments from people before installing anything.
A good place to start is 20 must-see Greasemonkey Addons