Firefox does not open .onion websites

It's the easiest and safest to use Tor Browser package, but if you need to prefer Firefox for some reasons, try it as follows ...

1. sudo vi /etc/resolv.conf

nameserver 127.0.0.1

# nameserver 127.0.0.53

    and verify if it's working in the way like ...

%dig @127.0.0.1 -p53 www.google.com 

2. #/etc/init.d/tor restart

    and verify if it's working in the way like ...

%ps -ef | grep tor

3. Configure network.proxy.socks_remote_dns true

3-1. Launch Firefox web browser, type about:config in the address bar and press Enter.

3-2. After clicking on the “I’ll be careful, I promise” button, proceed to Firefox advanced settings manager.

3-3. In the search box type network.proxy.socks_remote_dns to find the setting we need.

3-4. Make sure the value of this setting is set to true. If the value is false, then double-click on this setting to switch it back to true.

4. Configure proxy 

In the Preferences of Firefox, at Network Settings, click "settings" and configure SOCKS Host localhost Port 9050 (with Manual proxy configuration automatically activated) .

That’s it. Now Firefox will be able to open all sites that operate using the .onion top level domains.

FYI:

network.dns.blockDotOnion hasn't matter in my environment.

Both "true" and "false" can work with the socks proxy.

References:

Ref.1 Firefox does not open .onion websites 

Ref.2 https://akasaka-taro.blogspot.com/2017/10/tor-browser.html