Visible tables in Mozilla Thunderbird?

Question

Mozilla Thunderbird

How to make in Tbird so that when inserting a table into a letter (from Excel, for example), the borders of the table cells are immediately visible?

Answer 1

A simple hack: in the TB settings there is an opportunity to assign a signature to all letters - in HTML.
It is enough, in addition to the usual data (name and phone number of the manager), to insert such a block there, for example:

<style>
table td, table th {
  border: 1px dotted red;
}
</style>

Answer 2

Based on Adamos and according to userContent.css documentation :

table {
  border-collapse: collapse;
}
table th, table td {
  border: 1px solid black;
}