I am one of those persons that has their Firefox configured to not close when the last tab is closed, i.e., having toggled browser.tabs.closeWindowWithLastTab. However, having tabs pinned breaks this behaviour.
I am used to having an empty tab when I close all my tabs, so I can start working in an empty session on what I was planning to do. When I have a pinned tab, I unavoidably end up in that tab when I close all others. From there, I have to open a new tab manually.
The pinned tab behaviour does exist to have permanent access to some apps that should either run in the background or are accessed often (like mail, music). I think it makes sense to at least have the option to get a fresh tab when all (non-pinned) tabs are closed.
I turn on an option, where pinned tabs are no longer considered for the number of open tabs. When no tabs are pinned, behaviour is unchanged.
However, when the setting is on and closeWithLastTab is off, this happens: When I close the last (non-pinned) tab, the browser will think it has no more remaining tabs and opens a new one. Desired Behavour.
If this new setting is turned on, closeWithLastTab is on (default), and I have pinned tabs, then the windows closes even though there still are (pinned) tabs open, which might be unexpected. Therefore, the setting has to be turned off by default. But there might even be a new use case, where users have some sites pinned that are useful to them, like a knowledge base or something, but just to have quick access (somewhat like a bookmark). These users might now be able to close their browser by closing the last tab they are working with, because they do not care about their pinned tabs staying open.