Steps to replicate the issue (include links if applicable):

1. Use Mozilla Firefox (I tested version 140 ESR on Debian 13 stable)
2. Open the WikiEditor in a page like https://en.wikipedia.org/w/index.php?title=Wikipedia:Sandbox&action=edit
3. Make sure to use Syntax Highlighting
   • it's the pencil icon in the top-toolbar of the editor
4. Write something long, quickly, like "hello hello hello"

What happens?:

You type "hello hello hello" but you get rubbish like "helloh elol ehlol"

What should have happened instead?:

You type "hello hello hello" and you get "hello hello hello".

Workarounds

• disable the Syntax Highlighting
• or, use Chromium (version 142 on Debian GNU/Linux stable trixie, works)
• or, use Konqueror (4:25.04.0-1 on Debian GNU/Linux stable trixie, works)
• or, use Firefox 145 on Fedora

Screenshots

Note the green paragraph (what I really typed, when Syntax Highlighting was disabled)

Note the red part (what I got when typing a similar thing with Syntax Highlighting enabled.