For installing unsigned web extensions in Serpent 52 and 55 (moebius! ) they have first to be compatible with these browsers. Deleting any files from the folder META-INF of a xpi file does not do anything, and is therefore no solution. Furthermore, only setting the preference xpinstall.signatures.required to false, which is already the default setting in Serpent, is no solution, either. To install such an unsigned web extension permanently (there is no META-INF folder in its archive, or it has been removed) in Serpent, the file manifest.json has to be modified to assign a unique ID to this extension. For testing purpose, I did that in the case of the web extension save-as-ebook 1.4.2 loaded from https://github.com/alexadam/save-as-ebook/tree/master/web-extension which lacks of such an ID in its file manifest.json. After assigning a unique ID to it, I packed the complete extension into a zip file and renamed it to save-as-ebook-1.4.2.xpi. If done, it can be installed then as usual, works as it should and also survives the next browser restart. Without enabling add-on debugging which would only result in a temporary loading of the web extension anyway! As a unique ID, I chose user-save-as-ebook@email.com. Here is a screenshot as a proof:
Cheers, AstroSkipper