“Kill switch” is a bit dramatic. It’s an on or off toggle. Would be funny though to call every toggle a kill switch. “Yeah, using the kill switch on GPU acceleration may help with rendering on some systems.”

“Use the kill switch for preventing Firefox of starting a new session without restoring the old tabs.”

“Kill all of your browser data upon exiting Firefox by enabling the kill switch.”

“Make Firefox your default browser by enabling the ‘set as default browser kill switch’.”

Extended to other UI interaction classes: “You don’t like English? Kill it by using the battle royale language selector to choose only the one language you like.”