[Windows] Windows text replacement doesn't work on Firefox

[Witoso]

📝 Provide detailed reproduction steps (if any)

1. Enable text suggestions in Windows 10/11 (https://support.microsoft.com/en-us/windows/enable-text-suggestions-in-windows-0bf313ca-c992-4173-aa5f-8341d3953498).
2. Go to https://ckeditor.com/docs/ckeditor5/latest/examples/builds/classic-editor.html.
3. Type "thi".
4. Select "this" from the auto-complete box.

Was fixed on Chrome: #13583

❌ Actual result

The text replaces only the last character from the word, instead of the whole word.

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[urbanspr1nter]

Thanks @Witoso for creating this! Do you also feel this may be related to the former fix: #14117 -- Also noticed Burmese typing feels a bit broken.

[Witoso]

I think this one is not a regression caused by the fix. Most likely it's something that we missed and didn't work from the beginning.

[Witoso]

This could be reproducable on MacOS and Grammarly:

1. When on Firefox, go to the editor.
2. Scroll to Three monks ascending the stairs of an ancient temple.
3. Try to replace the ascending with ascended.
   
4. The word is not replaced: