This Firefox add-on is not endorsed by Facebook. They have no control over it.
Facebook has a 500 character limit for some messages, but you don't know if you're over the limit until you submit the text. This counter keeps track of your character count as you type.
A word on security / legitimacy, and a bunch of technical info.
Install version 1.2 (Tue, September 4, 2012) - Recent versions of Firefox (13.0.1 or before) will install as soon as you click the XPI file. Previously, one would save the XPI file as any other download, then do "File -> File Open" in Firefox and open it to install. The counter is free / open source, released under the GNU GPL.
Because the Facebook page takes time to process once downloaded, the counter may take a number of seconds (10?) and keystrokes (30? 50?) before it starts working. Similarly, it takes at least two keystrokes before the counter knows you've switched from "status" to "Ask Question."
Note that versions 1.1.3+ and 1.1.0+ have different internal IDs than previous versions, so this will not overwrite the previous. You'll have to uninstall previous versions manually. (I have no idea what will happen if they run at the same time. Almost certainly nothing tragic, but possibly something weird.)
Facebook may change their page such that this stops working. With that said, the Facebook page has not affected the counter for months, as of mid-July, 2012. If FB changes the page, it's usually very fast to fix this add-on, but I or another programmer needs to know about the FB change. I rarely use FB (perhaps oddly), so I won't know unless I'm told. My contact info is on my resume, linked from my home page, which is in turn linked at the top of this page. As for other programmers, this is licensed under the GPL, so anyone is welcome to fix it and propagate the fix. In fact, I'd be happy to know that someone else picks up the torch.
Sometime in the middle of 2011, the Facebook status message character limit went from 420 to 500.
