Custom host file for Firefox

You can use FoxyProxy Standard for that. It is free.

With FoxyProxy you can make that all requests to www.somedomain.com/* to be served by another ip address only for Firefox, without touching your hosts file, and the other browsers will be served by original/normal ip address.

It is very easy: You create a new proxy with "Add New Proxy", and in Details you assign your "other" ip address in "Host or IP Address" and the port, and later in the "URL Patterns" you enter for example "*://www.somedomain.com/*" and that is all. Only for Firefox the www.somedomain.com request will be served with the "other" ip.