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Custom JS Action not working in Tampermonkey and Firefox

Trying to add some custom styling but it doesn't work in Tampermonkey v4.5.5564 and Firefox 56. It worked in Greasemonkey 3.17.

I have Firefox 56 and it works here. alert("hi");

Try asking on Tampermonkey forum ( https://forum.tampermonkey.net/ ), maybe they just blocked custom js actions cuz it was not safe by their judgment

Are you able to use the code below? I can't. I think it's because of this https://stackoverflow.com/questions/2303147/injecting-js-functions-into-the-page-from-a-greasemonkey-script-on-chrome.

function addGlobalStyle(css) {
var head, style;
head = document.getElementsByTagName('head')[0];
if (!head) { return; }
style = document.createElement('style');
style.type = 'text/css';
style.innerHTML = css;
head.appendChild(style);
}
addGlobalStyle('body{background-image:url("chrome://global/skin/media/imagedoc-darknoise.png")!important;}');
addGlobalStyle('body>img:only-of-type{margin:8px auto!important;display:block;max-width:100%;height:auto!important;width:auto!important;}');

Are you able to use the code below? I can't.

but does alert("hi"); work fine for you? - if so then it's the problem of the code itself

document.createElement()

it's protected by the script against aggressive ads, try using protected_createElement(), it should be accessible from custom js action cuz it is executed from inside the script

alert("hi"); works, but the code I'm trying doesn't work. I tried using protected_createElement() but got the error TypeError: document.protected_createElement is not a function

You'r right, for some reason original createdocument works in js action,

and I've tested your original code - it adds two <style> elements to the head fine in my firefox