That looks totally fine, that should be the last line of log output just before the URL gets loaded in your browser. I tested on Linux Mint 18.1 with Cinnamon to try to reproduce this issue, but it is working fine for me both with Picard 1.3.2 from the repository and the latest 1.4 from source.
Maybe stupid question, but are you absolutely sure it does not load in Firefox? Maybe you have multiple Firefox windows open and the URL opens in a new tab in another window than you expect.
Yes, that’s an Picard internal module. It is an extension of webbrowser with some extensions. As a last test could you try to run this on the command line:
I think this is what actually should get run by Picard on Linux in most cases.