I guess Firefox registers itself as the default ftp:// protocol handler somehow. So Firefox tries to open the ftp:// link with an external app, which ends up with launching itself. If Firefox receives another unknown protocol, it will search the URL with the default search engine instead. I think ftp:// should behave similarly when network.ftp.enabled is true.
Bug 1673229 Comment 2 Edit History
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I guess Firefox registers itself as the default ftp:// protocol handler somehow. So Firefox tries to open the ftp:// link with an external app, which ends up with launching itself. If Firefox receives another unknown protocol, it will search the URL with the default search engine instead. I think ftp:// should behave similarly when network.ftp.enabled is false.