Q:

Firefox addon's content script will not run if there is a "return" statement anywhere

I'm working on a Firefox addon that utilizes content scripts to add some additional features to a website. If my content script contains a return statement, the code won't run - not even the "console.log" statement in the first line of the script!
I have tested this on Firefox 68.0.2 (64-bit). The return statement is in an if block, so any console.log statements before it should be executed - but they aren't. If I comment out the "return" line, all lines before and after it work again, but if I leave it in, not even the first statement is executed.
console.log("Starting XX-Tools")
var ctr_player = window.VIDEO_PLAYERJS
if (typeof ctr_player === undefined) {
    console.log("No player found")
    return
}
console.log("Player found!")

I was expecting:  
Starting XX-Tools  
Player found

or at least:  
Starting XX-Tools  
No player found

I got no output. If I remove the return statement, I get the following output:
Starting XX-Tools
Player found

A:

If that code is not inside a function, then return is a syntax error. – Pointy
See https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Errors/Bad_return_or_yield

