Can you explain where exactly you try to copy and paste? More info would be appreciated.

OK, so it turns out that this actually seems to be a Firefox Setting called dom.event.clipboardevents.enabled ! Turning this off in about:config made twitter work as it is supposed to, but I will look into it.
https://www.ghacks.net/2014/01/08/block-websites-reading-modifying-clipboard-contents-firefox/

I managed to re-implement Copy and Cut, Paste is still somewhat hard to reach, but I will keep looking. In the meantime I'll push the cut/copy update

Update: I managed to restore pasting somewhat. It's hacky, but it seems to work.