Insert horizontal line then move upwards breaks page on firefox

[Famiboards]

This is a bug that I can confirm for Firefox. It does not appear to happen on chrome.
I am using firefox 128.0 (64-bit).
I don't think this was a problem before 2.3

Steps to reproduce:
1. Add a horizontal line
2. Press up on the arrow keys

Result:
The page itself breaks causing render issues. I'm unable to do anything including opening up the developer menu. I have to close the tab and reopen.

Attached is a video demonstrating the problem.
Insert horizontal line then move upwards breaks page on firefox

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