Thank you for leaving feedback! I looked into the issue and I managed to get the glitchy behavior when copy/pasting text into a comment box. However, I also seem to be getting the same glitch even when the extension is disabled. I looked into it a bit and it seems to be a known issue with Firefox on Reddit's end.

The issue seems to stem from the "Fancy Pants Editor" and most of what I saw recommended switching to the "Markdown Mode" when copy/pasting text.

Here is a thread documenting the issue: https://www.reddit.com/r/bugs/comments/qec00j/copying_editing_a_text_is_broken_completely/