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  1. Firstly, proceed with the following instructions at your own risk. I assume no responsibility for lost data or broken using experiences as a result of this workaround. In general, it is unlikely that any irrevisible changes will happen as a result of importing an extension that has not been tested for your installed version of Firefox/Thunderbird, as you will most likely be able to just uninstall it if it does not work as desired.
  2. Download the *.xpi file using a browser other than Firefox. This seemed to be the only way that I was able to actually “Download Anyway” an extension.
  3. Using an archive manager, open the downloaded *.xpi file. I used Engrampa which is part of the MATE desktop, but that by no means is exclusive. Inside this archive, you’ll want to modify a file named install.rdf:
    <?xml version="1.0"?>
    <RDF xmlns="http://www.w3.org/1999/02/22-rdf-syntax-ns#"
    xmlns:em="http://www.mozilla.org/2004/em-rdf#">
    
    <Description about="urn:mozilla:install-manifest">
    <em:type>4</em:type>
    <em:id>{77d2ed30-4cd2-11e0-b8af-0800200c9a66}</em:id>
    <em:version>14.0</em:version>
    
    <!-- Target Application this extension can install into,
    with minimum and maximum supported versions. -->
    
    <em:targetApplication>
    <Description>
    <!-- Firefox's UUID // DO NOT CHANGE-->
    <em:id>{ec8030f7-c20a-464f-9b0e-13a3a9e97384}</em:id>
    <!-- End Firefox's UUID // -->
    <em:minVersion>44.0a1</em:minVersion>
    <em:maxVersion>45.*</em:maxVersion>
    </Description>
    </em:targetApplication>
    <em:unpack>true</em:unpack>
    
    <!-- Front End MetaData -->
    <!-- My_Theme -->
    <em:name>FT DeepDark</em:name>
    <em:description>Smooth dark theme for Firefox</em:description>
    <em:creator>Stefano</em:creator>
    <em:contributor>Stefano</em:contributor>
    <em:homepageURL>http://www.stefrosselli.com</em:homepageURL>
    
    <em:internalName>ftdeepdark</em:internalName>
    </Description>
    
    </RDF>
    

    Bump up the version of em:maxVersion to be inclusive of the version of Firefox/Thunderbird you are using, save the file and update the archive with the updated version of that file, and now manually import the extension as a file.

    In many cases, such as themes, this has worked for me well with the Nightly channels of Firefox and Thunderbird (50.0a). Below is a screenshot of the elementary Thunderbird theme installed successfully on Daily (50.0a).

elementary Thunderbird theme on Daily