After reading an article about 6 excellent FF extension, I had installed followings. Did not install other because Tab Mix Plus did a better job. :-)
1. Hyperwords - Select any word or words on any web page. A menu pops-up. Choose from any one of the many powerful commands. And that's it. Fully customizable menu and behavior.
2. Read it Later - This Firefox extension allows you to save pages of interest to read later. It eliminates cluttering of bookmarks with sites that are merely of a one-time interest. A commenter below (Chris) summed it up very well: “It’s a ’staging area’ for bookmarks.”