With Firefox 79.0 on Windows 7 64 bits, with the wiki editor, we can only undo/redo once pasting content that replaces all the current content.
This is especially annoying because it means there's no reliable workaround to T124435.

Steps to reproduce:

1. Open https://en.wikipedia.org/w/index.php?title=Air&action=edit in Firefox
2. Copy the last two lines
3. Press CTRL + A to select all the text
4. Press CTRL + V to paste the copied text
5. Press CTRL + Z to undo paste (it works)
6. Press CTRL + Y to redo paste (it works)
7. Press CTRL + Z to undo paste (it doesn't work)