It just started working again for me today in the latest Firefox (5.0.1). I'm on a Mac. Have you downloaded the latest extension?
I just upgraded to 5.0.1 (Win) (doesn't FF automatically encourage you to upgrade nowadays?), but I got the message that the Google extension isn't compatible with FF 5.0.1.
Maybe it doesn't matter. When it came out at first, I found all sorts of uses and misuses. I wrote to two bloggers and they took the pics down and apologised. I'd hoped they might license th image, but they were totally private bloggers, so not hopeful - but a result. Then we got the nippy note in the iStock forum about not contacting buyers directly, so then next six or so I wrote to Support about and have heard nothing since (apart from an acknowledgement of receipt of my support tickets). No surprise there. I guess if I find any more, I'll be contacting them directly, unless it's a commercial website.