I wrote up a quick-and-dirty “User Style” to make Twitter not suck so bad. Now it looks like this:
Here’s how to make it work on Firefox: Install Stylish and restart Firefox. Go to twitter.com and log in. Click the Stylish icon in the bottom-left corner, choose Write New Style -> For This Page. Give it a name of your choice and replace the text with the contents of this text file. Hit save. Enjoy.