Pros:
- Syntax highlighting in the editor
- Works on 3 levels: Globally, URL, and Domain
- Very lightweight. You won't even notice it's there!

Cons:
- It's really frustrating that I need to click "Save" before I click out of the editor or I lose my changes. While working on some CSS for Discord's Streamkit Overlay, I lost a ton of changes by accidentally clicking outside of the editor.
- Sometimes, the domain level CSS does not get applied or doesn't come up when you open the extension. I've had to refresh up to 5 times to get it to appear and no amount of closing and reopening the extension will fix it.
- For some reason, it keeps mixing up some of my domain-level CSS changes. Sometimes I'll have my custom CSS for a website where I study language applied to YouTube and vice versa for example.

Overall, it's a very useful extension that I find incredibly helpful for helping me study by adding things spoiler effects to flash cards and such.

Nice plugin!

Is it possible to get rid of the [CustomCSS Injector] Applied custom CSS. console.log() message?

Somehow, with Firefox 80 the EBay Print Shipping Label page was placing up/down arrows in the numeric input fields for weight and dimensions rendering them unusable. I was able to find the offending CSS and figure out how to defeat it. Then I am very easily able to use this addon to over-ride the offending CSS. Freaking great! But I am really only a novice in all these matters. At least this prevents me having to switch to Chrome for this page/operation. Thanks!

Doesn't seem to work at all. No matter what css rule I put in the css box, it's always highlighted red (as in invalid data) when I click save. No errors, no feedback, no nothing. Even error console doesn't produce any output at all.

Edit: Tested again after receiving developer's feedback and it seems to be working fine.