FIREFOX: Corners of the background not getting rounded despite the webpage container having rounded corners

[princess-wawa]

Describe the bug The corners of the webpage are straight while the box they're in has rounded corners, as you can see here

To Reproduce Steps to reproduce the behavior:

1. Install a userchrome.css that has rounded corners on the webpage part of the browser, or you can just turn the value of uc.tweak.rounded-corners to true in about:config
2. go to the regular new tab with the extension installed or to https://online.bonjourr.fr
3. you should see the corners sticking out of the webpage window

Expected behavior the edges of the new tab page shouldn't stick out of the webpage window

Screenshots I've managed to find what caused the issue, it's this CSS line: as you can see, when disabled

this is with everything else removed, but you can also see that it behaves the same with the full page shown

the closest I've been able to get it working without making the corners look like that is by changing the blur to a background-color: rgba(255, 255, 255, 0.2);

Device (please complete the following information):

• OS: Arch Linux x86_64
• Browser: Firefox
• Version: 131.0.3