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This entry has been published on 2018-10-13 and may be out of date.

Last Updated on 2018-10-13.

[:en]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.

Solution

Open regedit and delete

HKLM\SOFTWARE\Classes\FirefoxURL\shell\open\ddeexec\topic

(or delete the default value of this key).

Background

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)[:]