Manually setting it in about:config is not good enough.

When you connect to a network that needs its own DNS to access its resources, you are not going to change that setting each time you connect to that network.

Even more interesting thing happens, when you physically roam between several of such networks (for example: your company and your customer). What was until now handled transparently handled by DHCP for you, now you have to manually configure each time you connect to that another network.

Now, we have moved from static network config decades ago, and for a good reasons.