Users who have a lot of opened tabs might experience this issue. It's frustrating when you open a new tab from a pinned one, it will take you at the top of the tab list, and then you will have to jump between ~200 tabs it would be really harmful :(
Mozilla UI team wouldn't accept the behavior I proposed, saying that user don't use so much tabs, and this would be the right behavior. You can check this bug to have more details : https://bugzilla.mozilla.org/show_bug.cgi?id=970102
Here is an add-on that fixes the behavior when opening a new tab from a pinned tab. If you use a lot of tabs and pinned ones, it will make your day !
Report issues here : https://github.com/rednaks/appendedNewTab/issues