A lot of Chrome's extensions don't integrate well with the browser itself. They feel a lot like bookmarklets rather than an actual extension of the browser (this is by design). Firefox extensions can be very well integrated. For example, NoScript on Firefox far exceeds the quality any of the alternatives on Chrome.

Simply put, Chrome extensions are HTML/CSS/JS, while Firefox extensions can actually manipulate the interface of the browser to a greater degree (making stuff like Vimperator possible when it wouldn't be possible on Chrome)