They haven't taken it away from you...
After many retarded (jab at the topic's title ^^) requests Valve put it back into the game as a normal server variable but setting it to "on" will move you to the "custom" server tab - unless you use some kind of lame SourceMod plugin to hide config changes.
There were a lot of FF on servers back in the beta and although I'm sure it'd be funny as hell for video purposes TF2 simple does NOT work with FF on... It's not the way it's meant to be played which is why TF2 designers were reluctant to include such an option in the first place.
Funnily enough there were lots of FF on servers when Valve hid the setting but now there are hardly any at all. Just goes to show that some people only want to be jackasses and fight for "their freedom to do something" no matter how retarded it is. ;-)
I believe FF can be enabled using a plugin.
I'd recommend mp_friendlyfire 1...
P.S. McMoist - don't you remember how I turned FF on for a while to blow up Mint while we were playing on GD_concmap_v3? Or weren't you on the server at that moment?