@Gerard: i think there is another extension for you then: https://addons.mozilla.org/en-US/firefox/addon/origin-lock/ - there description sounds exactly what you asked for.

But personally for me - pinned is always pinned, i use it for things like issue trackers and wiki, where i want "table of contents" always open and never washed out, while all happend within the same site.

For other fellow reviewers, if you feel the feature of this one should be in the settings of out-of-the-box Firefox - vote https://connect.mozilla.org/t5/ideas/pinned-tab-should-not-all-the-url-to-change/idi-p/34397