In Firefox with WebExtensions works fine. Old FF like ESR - version 1.0 only.

Sample:
http://english-e-reader.net/book/two-worlds-antoinette-moses
Download book as TXT. When extension disabled (file download):

Content-Disposition: attachment; filename="Two_Worlds-Antoinette_Moses.txt"

When extension enabled (opened directly in browser):
Content-Disposition: inline; filename="Two_Worlds-Antoinette_Moses.txt"