I don't mean getting an addon as that was a total failure. How can I do it in about: config so that it actually works. I downloaded an app called phoney and all it did was slow down the browser and messes up the pages.

Go to the about:config page, filter for "useragent" (hit down and enter), and change it to what you wish.

Sorry, I've tried that but it doesn't give me the option.

Sorry, I didn't read your title; that's for mircrob not firefox.

In firefox go to about:config, right click "New">"String"

For the name, type:
"general.useragent.override"
and the value, whatever ever agent you want.

I can't right click on it. It won't let me.

Does holding down for a few seconds do anything?
If not try Control+n.

Oh, I see it. I need to add a new preference. Thanks. I think I got it.

Maybe I can do this by putting a new string to override the version of flash on firefox? Wouldn't know how to do this this though.