From MozillaZine Knowledge Base
Many UI elements in the Mozilla “chrome” can be accessed by using an access key with another key. (The appropriate key is usually underlined in the UI element’s label.) For example, the File menu can normally be accessed by pressing "Alt+F". However, web pages can also specify access keys that can conflict with Mozilla’s shortcuts. To avoid this problem, the access key used for web page content can be specified separately from the chrome. This preference determines which access key is used for Mozilla’s chrome.
Possible values and their effects
Disable the access key for the chrome.
Use "Shift" for the chrome access key.
Use "Ctrl" for the chrome access key. (Default in Mac OS X)
Use "Alt" for the chrome access key. (Default in Windows)
Use "Meta" for the chrome access key.
You can add any of the above values together to combine their functionality. That is, setting this preference to 3 (i.e., 1 + 2) means you need to press "Shift+Ctrl".
It’s a good idea to keep this preference set to a different value than ui.key.contentAccess for the reasons mentioned above.
First checked in
Has an effect in