Label backgrounds have too much padding on Linux + Firefox

[anne-gropler]

What happened?

When working on a sandcastle for another issue (#12138), I noticed that the labels look odd on a different machine. I was told by a collague that this issue exists for at least a year, that we know of.

The labels have too much padding on Linux and Firefox. Also note the weird scattered single pixels in the background:

They do look fine ( = as expected) on the same Linux but with Chrome. Well fitting padding and clear white-with-alpha background.

Reproduction steps

1. Select a machine running Linux.
2. Open Cesium Viewer in Firefox
3. add Labels (as entities or as label primitives) that use a background

Sandcastle example

https://sandcastle.cesium.com/#c=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

Environment

Browser: Firefox 134.0.1
CesiumJS Version: 1.125, but issue seems to be waaay older.
Operating System: Arch Linux with Kernel 6.12.10; Nvidia GeForce GTX 1660 Ti with Nvidia driver Version 565.77