Did it crash your browser?
It can actually take a while to take the screenshot. For example I've clicked 'add', and had to wait up to 5 seconds sometimes for the screenshot to actually be taken, but it usually does come back.
Can you try waiting it out? If it still doesn't work I'd love to know the FF version and OS version if you can share.
The UI has a bunch of little issues that I'm sure you'll get to. Also, the add screenshot button on the feedback windows froze firefox. But otherwise, pretty slice