With so many new browsers like Safari and Chrome, I still like Firefox mainly because of its extension. It is because the other browsers lack the "smart drag and drop" feature. This feature lets you just drag and link and open it in a new background tab. Currently I am using the Easy Dragtogo plugin with Firefox 3.1 beta.
I am also playing with a plugin called AutoPager. This plugin will auto load the next page when you reach the end of current page. It is particular useful when using with Google.
What Firefox plugin do you use?