The Mozilla Firefox web browser has a separate independent DNS cache which allows it to access the internet faster as Firefox retrieves the DNS information of a visited web page from the DNS cache. Sometimes, you may need todisable the DNS cache of Mozilla Firefox. When the DNS cache of Firefox has been deleted,it will use the cache of the operating system. To disable the DNS cache of Mozilla Firefox, the value of the preference name network.dnsCacheExpiration must be set to 0. Preference names can be accessed from the browser window.
The Firefox web browser contains its own DNS cache, regardless of your operating system. For most users, it is quite convenient as it allows a faster web browsing since it doesn't have to submit a DNS request each time it connects to a recently visited website.
However, this can be very troublesome when you wish to set up manually the IP address of a server (e.g. to test purposes via the host file).
To disable the DNS cache:
Open your browser
Type in about:config in the address bar
Right click on the list of Properties and select New > Integer in the Context menu
Enter network.dnsCacheExpiration as the preference name and 0 as the integer value
When disabled, Firefox will use the DNS cache provided by the OS.
By default, a new version of Firefox will use the DNS information provided by the operating system and ignores its own cache
To empty this cache, without disabling it, just go to Tools > Delete recent history and set the interval to "Everything" and check "cache"