From MozillaZine Knowledge Base
The HTML 4.01 specification defines the
Possible values and their effects
Disable the accelerator key for the content.
Use "Shift" for the content access key.
Use "Ctrl" for the content access key.
Use "Alt" (Option on the Mac) for the content access key.
Use "Meta" (Command on the Mac) for the content 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". In Windows and Linux, the default is 5 ("Shift+Alt"). On macOS, the default is 6 (Control+Option).
It’s a good idea to keep this preference set to a different value than ui.key.chromeAccess for the reasons mentioned above.
First checked in
Has an effect in