This is a very basic Firefox extension that modifies the canvas_session cookie set by the Canvas LMS to not be a session cookie any more. The end result is that Canvas will stay logged in across browser sessions.
The extension expects that your Canvas instance is at a URL that begins with https://canvas.
Download the .xpi file from Github releases (it's just a zip of the repository contents). Because it's not signed by Mozilla you'll need to set xpinstall.signatures.required to false in about:config. It's not clear to me if all Firefox variants even have this option, though.
The Canvas mobile app already stays logged in forever, so this surely can't be too bad.