[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 two https://docs.google.com/spreadsheets and two https://www.office.com/launch/Excel
2- Create a table in both editors and give format to borders (change style and color)
3- Select table from google docs and copy
4- Paste it in the other google docs spreadsheet.
5- Notice that the borders don't match
6- Now select table from onedrive and copy.
7- Paste it in the other onedrive spreadhsheet.
8- Borders match perfectly.

[Expected result]
the table's format should match

[Actual result]
Format changes.

Note this is different from https://bugzilla.mozilla.org/show_bug.cgi?id=1641019, since it's not from one editor to a different editor, but within the same editor

The issue is still reproducible, however it also happens with Chrome. Therefore, not a Firefox-specific issue.