Scrollbar doesn't auto-hide on Firefox in fullscreen mode

[hyperjuni]

System:
Windows 10
Firefox 145.0.2

Steps to reproduce:

• Have a clean Firefox 145.0.2
• Have the Custom Scrollbar plugin installed, with the 'Automatically hide scrollbar' option turned on
• Open a webpage that allows for some scrolling (such as a YouTube video in Theater mode)
• Make the browser fullscreen by pressing F11

Expected behavior:

• The vertical scrollbar auto-hides no matter where the mouse cursor is, like it did up until recently

Actual behavior:

• The vertical scrollbar stays on-screen permanently
• The scrollbar auto-hides only when you hover over the titlebar

P.S. - The issue is (still) reproducible on the newest version of Firefox, 149