If usefull to anyone, here's how I solved this problem. Facts: Since FF27, I couldn't choose where to download a file nor send a file because the "browser" button wouldn't work. On last Avast programm update, I stopped "Avast Secure Line" from loading (thanks to my Outpost firewall) and then FF started working fine again. I was surprised because I had already tried all helps given including running FF with both Avast and Outpost stopped with no success. Hope this may help. All the best