Under Win 8.1, copy, one or several Spreadsheets cells - version 18.104.22.168 -, and pasting them into a new Thunderbird message (version 24.3.0), it does not work.
A paste without formatting works.
A paste of a cell content works.
It perfectly works under Linux CentOs 6.5 final release with the same Thunderbird and Spreadsheet versions.
Thanks for reporting this.
This behaviour was previously reported in
Bug 74584 - Can no longer copy spreadsheet cells from libreoffice calc to another app like thunderbird and paste as html table.
and is already fixed.
I set status for your report as DUPLICATE.
*** This bug has been marked as a duplicate of bug 74584 ***
Sorry for the duplicated bug. It is perfectly the same issue. ... I've search carefully but didn't find this one.