Dragging text in textarea is buggy (Firefox)

Hi,

I’ve got Firefox 89.0.2 on macOS and I’m experiencing a tiny-but-annoying bug in Github web UI. While editing some text in a textarea (eg a PR comment) I like to drag-and-drop text fragments. For example I have Hello world my dear in my Github textarea and then I select my dear, grab it and drop right after Hello. I expect to get Hello my dear world but the actual result is Hello my dear world my dear which is strange.

I cannot reproduce it in non-Guthub textareas so I assume it is somehow connected with the Github web client code.
Also this scenario works fine in Chrome and Safari, so the bug is Firefox-only or so.