Grey, opaque background on Windows 7 titlebar

[diogodiogo2]

Describe the bug

Since version 8.6.3 (ESR edition), the background appears as opaque grey instead on the intended Aero background on Windows 7.

It even happens in custom themes, where Aero isn't even enabled. In my dark theme, which has a dark background, this gray rectangle appears instead of the intended dark grey, the same way seen in the screenshot above.

Expected behavior

The intended behavior, seen up to 8.6.2, is a transparent, Aero background.

Screenshots

No response

OS

Windows 7

OS - Others

No response

Firefox Version

115.17.0esr (64-bits)

Distribution

• Original Lepton
• Lepton's photon style
• Lepton's proton style

Theme

• Light
• Dark
• Alpenglow (System: Light)
• Alpenglow (System: Dark)
• System Default
• Others: (e.g. Photon Colors, Google Chrome Dark, Fairytale Of Nature)

Theme - More Info

Issue also apparent on the following custom theme: https://addons.mozilla.org/en-US/firefox/addon/dark-theme-for-firefox/
For system default, Aero is enabled, as well as transparency.

user.js setup

Default user.js used

Additional context

No response