[petepete]

I don't think you need an add-on for this if you're already handy with CSS, just edit userContent.css and use a domain-specific query:

    @-moz-document domain(https://news.ycombinator.com) {...}

[markussss]

I find it incredibly useful to be able to toggle styles on and off in a simple GUI, especially for these two simple global styles:

    video { /* flip video horizontally */
        transform: scaleX(-1) !important;
    }

or

    /** scale videos that were originally 4:3, but 
        were encoded as 16:9 into proper 4:3 again */
    video {
        transform: scaleX(0.75) !important;
    }

It's not extremely useful, but every time I toggle them on, I'm very happy that I added them both.

[the_pwner224]

Out of curiosity, what do you use the first flip horizontally style for?

[markussss]

To flip videos that have been flipped, or for watching a video in the mirror. Maybe it's 1 in 100 videos, or even less, but it's so nice to be able to just toggle a button that flips the video properly.

[bastawhiz]

Not OP, but there's a lot of unscrupulous videos out there that flip video horizontally to evade content matching algorithms, and plenty of videos taken on phones or web cams that get flipped as well

[pvorb]

Most video call software will show your own webcam stream flipped horizontally to make it feel like a mirror, but others will see it correctly.

[butz]

You can even customize browser UI using userChrome.css to revert new style of location bar or remove some items from context menus, and even more. This guide should give you some ideas: https://www.userchrome.org/what-is-userchrome-css.html

For more inspiration and help visit https://www.reddit.com/r/FirefoxCSS/

[OJFord]

I would do this so much more if it didn't require a restart of Firefox to take effect. As it is, I also use Stylus.

[antman]

Did not know about that feature, I am impressed Mozilla has not blocked it in favor of some reason.