Display warning on specific domain with firefox

Question

Using Firefox 87, I would like have some kind of warning when I am on a specific website or domain. Like a parental control, except that instead of blocking the content, it just gives me a visual feedback that I am on a domain of my list.

For context, I am developing with Qt 6 and often end up on Qt 5 documentation when looking for help (https://doc.qt.io/qt-6/ vs https://doc.qt.io/qt-5/ ) which is sometimes (but not always) deprecated. So I would like to know easily that the page I'm looking for is not Qt6

Answer 1

You could use this addon to add some custom CSS depending on the URL :

https://addons.mozilla.org/en-GB/firefox/addon/custom-style-script/

e.g. something like :

if (document.URL.includes("qt-5")) {
    var css = 'body { background: red; }',
    head = document.head || document.getElementsByTagName('head')[0],
    style = document.createElement('style');

    head.appendChild(style);

    style.type = 'text/css';
    style.appendChild(document.createTextNode(css));
}