nickmcummins - 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.
- 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.
- 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:maxVersionto 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).
