The simplest browser extensions are almost always the greatest browser extensions. And TweetFired is no exception. Its one simple function is to bestow you with the most important advice the internet can offer: never, ever tweet.
Because we are all a bunch of blithering, attention-starved idiots, our Tweets tend to reflect this. Some hide it well. Others let slip a string of bigoted crap on the internet masses. So to remind you to pause before you Tweet (advice we should all heed), TweetFired simply replaces the compose window's "What's happening?" with a far more advisable "Remember: you are always one tweet away from being fired."
Advice these people probably could have benefited from.