From past few days, some users are unable to copy and paste text in Whatsapp Web in Firefox browser. This seems to be a bug in whatsapp web.
You can copy paste images without an issue, but not text from other tabs or notepad etc., however copy paste withing the whatsapp tab works fine.
Solution:
To solve the issue, simple use Google Chrome browser or install this addon:
https://addons.mozilla.org/en-US/firefox/addon/absolute-enable-right-click/
The extension is useful, especially when websites have disabled the copy and paste functions.
Hopefully, Whatsapp would fix the issue in future updates.