Reason
Although "World Wide Web" has become synonymous with "Internet," the two terms actually refer to different things. The Web is, in fact, only one of the communications methods that make up the Internet. Although "Minecraft" servers are on the Internet, they are not on the World Wide Web. When you try to connect to a "Minecraft" server using a Web browser like Firefox, the two systems are unable to communicate with each other. To access a "Minecraft" server, you must connect to it from within the game itself.
Connecting
To connect to a "Minecraft" server, launch the game and log in to your account. Click the "Multiplayer" button on the main menu screen. Type or paste the address of the "Minecraft" server into the text box directly beneath the "Enter the IP of a server to connect to it" heading. Click the "Connect" button and wait for "Minecraft" to connect. Some servers are private; only pre-approved users can connect to these servers.
Firefox
Although you cannot access "Minecraft" servers themselves through Firefox, you may be able to open a webpage associated with the server in your browser. If the server's address starts with "server," remove "server" from the IP address and try to connect to it in Firefox. If the server's address ends with a colon followed by a series of numbers, remove these as well. Connecting to the server's webpage, rather than the server itself, often allows you to, for example, read the server's gameplay rules or apply for access to a private server.
Finding Servers
Websites like MinecraftServers.info, MinecraftServers.net and the official "Minecraft" website at Minecraft.net host lists of severs to which you can connect. Be aware of the server rules before connecting to a server. Some servers focus on peaceful collaboration, providing a space in which players can join together to build complicated structures. Others treat "Minecraft" more like a first-person shooter; connect to one of these "player versus player" severs if you want to fight other players instead of constructing buildings.