Well, it's not *wrong* but from where I'm sitting, add-on is technically correct, because it's something you add on; it's sort of converting between a noun and a verb.
Whereas 'addon' is neither, and in fact in some parts of the UK, it's a corruption of the verb 'to don' as in to wear.
Add-on is, in addition to being 'correct', is also more accepted since it's what Firefox uses (even if their site is addons.mozilla.org)
I'm not actually that bothered; I'd just always write it with the hyphen because to me that's how I think it should be, and why I said about not being a hyphenazi ;)