You are right I was one of the victim of such malicious script injected through firefox add-on. When ever I tried to paste a bitcoin address, got pasted different bitcoin address in blockchain wallet's sent field and lost around 0.005 bitcoins in 2 to 4 transactions before I noticed the problem.

I checked another member also had shared similar problem here in this forum at that time but with no concrete solutions, but I just resolved this by uninstalling my browsers.