Firefox itself does not provide a way to view DNS records of the websites you visit. However, you can install an extension to do so.
This add-on allows you to view A records, AAAA records, see the name servers and mail servers and all the other DNS records for any domain name or subdomain.
Follow this step by step guide to be able to easily view DNS records in Firefox.
In Firefox, click here to visit the Firefox add-ons page for the NsLookup extension.
Click the Add to Firefox button to download and install the extension.
Firefox will prompt you to confirm that you want to add this extension to your browser. Click Add to add the extension.
Open a new tab and visit the website that you want to view the DNS records for.
In the top-right corner of your browser, click the 'N' icon. This will show you the DNS records for the domain name of your active tab.
You can now see the DNS records for the domain you just visited. If the record type you're looking for is not shown, scroll to the bottom to select it from the drop-down.