Works well. (For any new user: It applies to https://www.linkedin.com/messaging only. NOT to the "Messaging" popup near the bottom right corner.)
One warning to the users, and a suggestion to the author: While on addons.mozilla.org this is called "Linkedin Messaging Boost", once you install it, in your Firefox's list of installed extensions (special URL about:addons) it's called "Linkedin-Mod".
