Firefox 87はなぜサポートされなくなったのですか？

[Benjamin_D (Benjamin Decotte)]

This will probably help:

@david

@sam

My guess here @david is that this may be related to the undo support?

Yeah, via MDN, Firefox < 89 had bugs with text manipulation:

Before Firefox 89, manipulating the content of <input> and <textarea> elements using Document.execCommand() commands requires workarounds (see bug 1220696).