I am using the latest Mozilla Firefox and the latest Gmail.
This extension works as described,
HOWEVER
Printing does not work if the email series is longer than one page of print.
What happens is that a full page of blank (with a top heading email line) is printed and then a second page is printed and the expected subsequent pages of print are missing.
This printing error does not occur with any other printing I am doing.
A scanned example is available upon request to the author.