User Agent: Mozilla/5.0 (Windows NT 10.0; Win64; x64; rv:63.0) Gecko/20100101 Firefox/63.0 Steps to reproduce: Clicking internal navigation within the website www.dreamwidth.org that change the subdomain. First case: click link that goes from the subdomain good-girl-elly.dreamwidth.org to www.dreamwidth.org. This case works correctly, link is recognized as being the same website and not open in a new tab Second case: click link that goes from the main domain www.dreamwidth.org to the subdomain good-girl-elly.dreamwidth.org. This case dose not work correctly. Pinned tab functionality reference https://support.mozilla.org/en-US/kb/pinned-tabs-keep-favorite-websites-open#w_how-are-pinned-tabs-different-than-normal-tabs "Links to other websites open in a new tab so that your Pinned Tab doesn't change." Firefox version 63.0.3 Actual results: Link is opened in a new tab, as the subdomain is incorrectly interpreted as being a different website. Expected results: Link should open in the same tab as the domain and subdomain are both the same website.

A subdomain can be an entirely different site. It depends on the site. :/