From MozillaZine Knowledge Base
The key used to focus/activate various elements in Mozilla is determined by this preference. Until very recently, this preference affected both chrome and content-area elements.
Possible values and their effects
Disable the access key.
Use the "Ctrl" key as the access key. (Default in Mac OS X)
Use the "Alt" key as the access key. (Default in Windows)
All possible values are defined in
First checked in
Has an effect in