Different Language in FF

Hello,

at first..., sorry for my terrible english :-D.
I use FF for and now I've bought an USB-Stick for myself.
I like FF and I use it on my computer with the german language.
Now I get PFF :-), it's a fine program and I have no problem to use it with
the english language, but I'm to lazy.
I want to know, how I can change the language of this really good program?

Greetz

McLeod

PS: I'm also using Thunderbird as well