Done a clean install on my laptop and suddenly firefox doesn't open magnet links anymore. Here's how to enable it again:
1. Open Firefox and type in about:config in the Address Bar and hit Enter.
2. Type in enter handler.expose in the search box at the top of the list.
3. right click - New - Boolean
4. Enter the preference name network.protocol-handler.expose.magnet
5. Set its value to false
6. Click on the magnet link and you should see Firefox’s Launch Application Choose Dialog
7. Select your torrent client.
Got it from here.