I tend to leave a lot of comments on various blogs that I read throughout my day, and typing in the same name, email and site URL takes quite a bit of time if you don't automate it somehow.

I've found a Greasemonkey script that will let you easily automate this process in Firefox so that you can just hit a hotkey to fill in those values every time. You'll need the Greasemonkey extension installed for this to work.

After you install the script, go to any blog page with a comment form. (If you have a page already open during the install you'll need to reload it) Now hit the Alt+C key combination, and you'll immediately be prompted to enter in the default values.

Once you use these values once, they'll be stored for next time, and the comment fields will be filled in. If you ever want to change them, just use Shift+Alt+C instead.

This should work for the majority of blogs, but not necessarily every blog, since it's made to work with Wordpress.

Download Pre-Fill Comments from UserScripts