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<a href="https://mywebsite.com/" rel="sidebar" title="A title">Add to bookmarks</a>
It seems these things were browser specific. I wonder if this feature can be properly detected on Mozilla-based browser. Looks like checking for existence of window.sidebar in JavaScript doesn't guarantee the feature is supported and in this case the link will act as plain link to the web page.
There's nothing interesting in window.sidebar in Pale Moon, looks the same as window.external. Maybe the only realistic way these days is using something like navigator.userAgent.includes('Goanna') for UXP browsers. Maybe there's another reason user agent sniffing was (is still?) popular?