Wasn't sure if you'd already found this (you've gotten "as far as the xpi installation file"), but...
The .xpi files live in the 'extensions' folder in your profile:
C:\Users\{yourusername}\AppData\Roaming\Moonchild Productions\Pale Moon\Profiles\********.default\extensions
Quickest way to get there is via Pale Moon's Troubleshooting page (about:support): Help -> Troubleshooting Information
Click the 'Show Folder' button to open your profile folder.
That same Troubleshooting page lists all the extensions you've installed, including IDs (which correspond to the file names in the 'extensions' folder) -- So use this list to identify the files.
(The one "extension" that you found in your PM installation directory, BTW, is actually the browser's default theme.)
Thank you, no that is the problem. I have the installation xpi. files in a folder where I put downloaded software, but then, where Windows hides them after installation usually is a big mystery. I have looked up in the programme folder of Pale Moon and didn't find anything except for that one extension which you explained as the default theme. The add-on list on the trouble shooting page gives the IDs, but nothing else. From that point I got lost. So, thanks for your guide.
