In some special cases it might happen that HTTP(s) links open as duplicates in separate Firefox browser windows, if they are contained in an Outlook email, Explorer shortcut (.lnk) etc. This behavior does not happen having another browser set as default, like IE.
Open regedit and delete
(or delete the default value of this key).
Some Firefox setup procedures seem to set this key for whatever reason. E.g. v61 does not set it, v55 does set it.
Even if you upgrade to a newer version, this key is not removed automatically! (But at least it is not re-set in newer versions)