If you can’t copy and paste properly on Facebook without the comment or message becoming jumbled up, you may have to allow a certain Firefox setting in the about:config that allows clipboard events to function properly on Linux distros.
Open a new tab and type:
about:config
Search for:
dom.event.clipboardevents.enabled
Double click on this entry to set it to true and copying and pasting correctly from the clipboard should function as expected now.
This fixed the issue for me when I could not copy and paste on Facebook messenger or comments on Firefox.