Firefox: Split divider is unresponsive in Multi Editor Radio Game

Describe the bug
When opening the Multi Editor Radio Game page in Firefox, the vertical split divider cannot be adjusted. Dragging the divider left or right has no effect, making it impossible to resize the editor panels.

Steps to reproduce the behavior:

1. Navigate to https://makecode.microbit.org/beta in the Firefox browser.
2. Scroll down to the Radio Games section on the home page.
3. Select Multi Editor and click Show Instructions.
4. Open the Multi Editor page at https://makecode.microbit.org/beta---multi#.
5. Attempt to drag the vertical divider left and right between the editors.

Actual behavior
In Firefox, the divider does not respond to dragging and editor layout remains fixed and cannot be resized.

Expected behavior
In Firefox, the divider should be draggable and function normally, consistent with the behavior in Chrome and Edge.

Screen recording
https://github.com/user-attachments/assets/5713dc0f-b3a2-4307-8496-a462ce17d328

Additional context
1.OS: Windows 11 25H2
2.Browser: Firefox
3.makecode.microbit.org version: 8.1.10
4.Microsoft MakeCode version: 12.3.5

Start by reproducing the issue at https://makecode.microbit.org/beta---multi# in Firefox, following the listed Radio Games and Multi Editor steps. Trace the vertical split divider used between the editors and compare its dragging behavior in Firefox with Chrome or Edge; done means the divider resizes both panels in Firefox as expected.