Currently in review. Get it from Mozilla at: https://addons.mozilla.org/en-US/firefox/addon/timeline-toggle/
A Facebook button with a giant X over it (in the bottom right side of your browser screen) will show up after installing. If you click this button while you're signed into Facebook, your timeline will be hidden! If you click it again, it'll be back. This way you can keep Facebook open to message people, but you won't get distracted by your friends timeline updates streaming by forever.
If you know how to program, feel free to make improvements and submit pull requests!