[Affected versions]
Release 77.0 (64-bit)
Beta 78.0b5 (64-bit) (64-bit)
Firefox Nightly 79.0a1 (2020-06-10) (32-bit)

[Affected platforms]
( macOS 10.14 (Mojave))

[Steps to reproduce]

1- Launch the Firefox Browser and open an online editor https://docs.google.com/spreadsheets and also open installed app (libreoffice)
2- Create a table in online editor spreadsheet
3- Select non-consecutive columns and copy
4- Paste it in the same sheet or installed app.
5- Columns are pasted next to each other (it doesn't matter how many columnes were in between that were not copied)
6- Now create a table in installed app.
7- Select non-consecutive columns and copy
8- Paste it in online editor.

[Expected result]
Table should be pasted accordingly.

[Actual result]
Notice that the columns are separated by as many columns that were present in original table that were not included in the selection.

This issue is reproducible on Nightly 80.0a1 (2020-07-22)(32-bit). I'll change flags accordingly.

The "issue" doesn't occur on Ubuntu 18.04 with current Nightly when copying from Google Spreadsheets. It does occur, when copying from LibreOffice Calc, but the latter is free to export what it wants; this is not an issue in Gecko.

Side note: Google Spreadsheets fills the clipboard with their own JS.