I don't want to say it's 100% Duolingo's fault, but I've noticed in the last few days when I've been doing lessons, I've had Firefox crash on me while using Duolingo. Each time it has been while I had it as an active tab.

Like I said, I don't want to blame Duolingo Entirely, because I'm not 100% sure, but it is a common theme during each of my crashes so far (about 4) that I thought maybe I should mention it just in case it is a problem on your end that you can fix.

I'm on Firefox 23, Vista Home Premium for what it's worth.