Counting the number of words, characters without any extra clicks. With the Word Count Tool you can select the text and the statistics is shown in the status bar widget. It also gives detail report about the numbers of sentences, paragraphs, average lengths, and keyword density.
Here is more about this add-on from the developers:
This add-on reports the number of words, characters instantly without any extra mouse click. When you select the text of interest, the statistics about the number of words and characters of the text is described in the bottom right of the add-on widget that is similar to status bar (in case your widget is turned off, you can turn it on by going to the menu View/Toolbars/Add-on Bar or use the hot key: Ctrl+/ (or Cmd+/ in Mac OS)). If you opt to hide the statistics in the status bar, the number of words and characters then are shown by a tooltip by hovering the mouse on the tool icon.
Want to learn more? You can pick up the Word Count Tool add-on from the Firefox Add-ons website.