What OS? To make sure you have the right path to file...
In most OSes, the format is:
127.0.0.1 dumbsite.com
127.0.0.1 anotherdumbsite.com
You should be able to use ping or nslookup to verify that it works. In Windows and most *nix systems and on OS X it's effective as soon as you save the file.
Browser doesn't matter.