Starting with release 45 the Firefox browser (https://www.mozilla.org/en-US/firefox/45.0/releasenotes/) is actively blocking .onion domains.
You can fix this by editing the Firefox config manually:
- access the config by typing „about:config“ in the browser address bar and pressing the enter key
- search for „dotOnion“
- change the value of „network.dns.blockDotOnion“ from true to false