Facebook restricts the number of characters that users can enter to 420, it’s less restricting than the 140 characters of Twitter, but what the Facebook interface seriously lacks is an integrated character counter that can tell users how many more characters they can enter. The Facebook Status Counter is an extension for Firefox that integrates a character counter right in the Facebook UI.
Once installed, it will have no effect whatsoever on how you view your Facebook page, when you start typing in your status box, the counter will be appear under the status box telling you how many characters are left. This means you’ll never see that annoying little error box telling you you’ve exceeded the character limit.
Install Facebook Status Counter For Firefox
Update: This extension has been discontinued and removed from the Firefox Add-ons page.