This problem usually happens only on Firefox. It’s usually because of blocking by AntiVirus or Firewall installed on your computer.
It’s because some AntiViruses or Firewalls only allow verified add-on installations from addons.mozilla.org.
SO you can solve this by download and install itmanually. Here’s how to do it:
1. Right clickon the Install to Firefox button and then click the Save Link As menu.
2. Save the virol.xpi fileto your computer.
3. Open the Tools -> Add-ons menu or type about:addons in browser.
4. click on Gear button and then choose Install Add-on from File.
5. Choose the virol.xpifile that you have saved earlier.
6. You’ll get an installation confirmation window. Just click Add to continue.
7. If the process is a success, you’ll get a message telling you that the Add-on has been added to your Firefox.
