Firefox - accent color on close button is grey after switching to another app

[Angedestenebres]

Hey 🐈

As said in the title, I noticed this since the version 119 of Firefox when title bar is disabled:

The close button should be blue in my case.

I tried on 3 computers on Linux Mint Cinnamon 21.2, I see this little display bug.
I tried with the title bar and I didn't see this issue.
Also, when the icons is grey, when you move the cursor on it, the color comes back!