[BUG] For Firefox on Linux (with X11), event.shiftKey and event.ctrlKey are not being detected

[npn225]

1. Problem Description
   The shift key and ctrl key do not work for this script for Firefox on Linux. After some investigation, the cause seems to be that the event.shiftKey and event.ctrlKey in the script are not being detected. This means that many of the keyboard shortcuts do not work for this script. For example, shift+M should flip the video vertically. But when it is pressed, only the M key press is detected and the screen is flipped horizontally, instead.

2. URL where the above issue occurs
   This issue occurs on all webpages where this script is enabled! In addtion, this link here might shed some light on what exactly is going on, why it's happening, and how to possibly resolve it.

3. The browser and browser version information you are using
   I'm currently running Mozilla Firefox 131.0.3

4. Other additional information
   Operating System: Arch Linux
   Kernel: Linux 6.11.3-arch1-1
   Windows Manager: X11