Besser Pinned Tabs keeps a pinned tab showing what you pinned it for. Type an address for another site into a pinned tab and it opens in a new tab, rather than replacing the page you wanted to keep in place. Firefox already protects pinned tabs fairly well on its own. This add-on fills in the one gap it leaves, and does nothing else.

Type a new address in a pinned tab and, if it leads to a different site, it opens in a fresh tab. That way, the pinned page stays where it is.
The extension identifies pinned tabs on launch - so it knows which tabs to protect as soon as Firefox reopens.
No data collection of any kind, and the source code is public.

There is nothing to set up. The extension's preferences page holds only an optional debug log: switching it on records what the extension does and lets you download the result. Clicking the toolbar icon opens an About page describing the extension and the limitations below. The Chrome version of Besser Pinned Tabs covers three cases:

Closing a pinned tab by accident. If a pinned tab is accidentally closed with a keyboard shortcut, the extension instantly reopens it.
Smart link handling. When you click a link in a pinned tab that leads to a different website, the extension opens it in a new tab. Your pinned tabs keep their original content.
Navigation protection. If you type a new URL in the address bar of a pinned tab, the extension opens that site in a separate tab, again preserving the content of the pinned tab.

The Firefox Browser already handles case 1 and 2 natively. Therefore the Besser Pinned Tab add-on only needs to handle case 3 in order to provide a full protection of the pinned tabs. These points are the result of what Firefox permits an extension to do. They are not faults in the extension, and they cannot be fixed from the extension's side.

The navigation is caught as it starts loading rather than beforehand. The new tab is opened and the pinned tab is sent back to its own address, which makes the pinned page visibly reload, losing scroll position and anything unsaved.
Sites are matched by the last two parts of their domain name. This lets a pinned tab for google.com also handle mail.google.com, which is usually what you want. However, it also means unrelated sites like bbc.co.uk and tesco.co.uk both match co.uk. Between those sites, a typed address loads in the pinned tab instead of opening a new tab.
Firefox's own pages (about:, and similar internal addresses) are not covered.
The browser's Home button is not intercepted, because Firefox does not report it in a way the extension can act on.

Open source, no data collection: https://github.com/gerardkieffer/besser-pinned-tabs-firefox