[Affected versions]:
Release 76.0 (64-bit)
Beta 77.0b8 (64-bit)
Firefox Nightly 78.0a1 (2020-05-26) (64-bit)

[Affected platforms]:
Platforms: ( macOS 10.14 (Mojave))

Steps :

1- Create a simple Table with content in installed app (LibreOffice) or launch the Firefox browser to create a table in an online editor.
2- Select as many cells as desired, then right-click copy
3- Try to paste the selection from the Excel app into the online editor (or within the same sheet from online editor in a different cell)

Expected Results :
Selection should be copied successfully.

Actual Results :
"These actions are unavailable using the Edit menus" message is displayed instead.

Note: Please note that this issue does not occur in Google Chrome
Also, this happens using "edit" from menu as well. (edit/copy works, but edit/paste won't)

From an end user's perspective it's not obvious, but this is not a bug. See https://support.mozilla.org/en-US/questions/1196485 for details.