Fix README links

README.md

 The extension will add an icon next to your address bar. Most of the time, this icon will stay unchanged, and clicking it will show
 a message saying that no replication of the current tab's website has been set up by RWB.
 
-![Screenshot 1](https://raw.githubusercontent.com/babolivier/collateral-freedom/master/screenshots/firefox/sc1.png)
+![Screenshot 1](https://raw.githubusercontent.com/babolivier/collateral-freedom/firefox/screenshots/firefox/sc1.png)
 
 When connecting (or trying to connect) to a website for which a replicated version has been set up by RWB, the extension's icon will turn
 red. Clicking it will open a pop-up page with a button.
 
-![Screenshot 2](https://raw.githubusercontent.com/babolivier/collateral-freedom/master/screenshots/firefox/sc2.png)
+![Screenshot 2](https://raw.githubusercontent.com/babolivier/collateral-freedom/firefox/screenshots/firefox/sc2.png)
 
 If you click the button, it will bring you to the replicated website.
 
-![Screenshot 3](https://raw.githubusercontent.com/babolivier/collateral-freedom/master/screenshots/firefox/sc3.png)
+![Screenshot 3](https://raw.githubusercontent.com/babolivier/collateral-freedom/firefox/screenshots/firefox/sc3.png)
 
 ## Contribute