Trying to install the nessessary enhancement "IE.Communicator.exe" results in...
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IE Communicator
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New version is available. Would you like to update your OWIE Communicator from official site?
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OK
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... only possible option (ok) results in ...
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Fehler
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Could not open key because it is missing. You may want to switch to "Custom" path option to specify executable path manually by extension options page.
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OK
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... Bullshit!!! Results in... Delete "Open with Internet Explorer"-Add On:-( from my Firefox

Although you can install this add-on in Firefox 57, don't bother. It puts an icon in the toolbar, but it does absolutely nothing. There's no option in the context menu either. I'm very disappointed and a little angry.

Practical addon, thank you!