As Alex did also mention in comment 5, to successfully install non-temporarily an unsigned extension the user has to make sure that an addon id is assigned through the manifest (on the contrary an addon id is generated automatically when the extension is installed temporarily, if one is not explicitly set in the manifest.json file). I'm pretty sure that the issue reporter is very likely related to that (trying to install non-temporarily an addon without an explicit addon id in the manifest.json file). I think it would be reasonable to explicitly mention that the addon id has to be specified in the manifest to successfully install an unsigned extensions in the doc page linked in comment 0 (https://extensionworkshop.com/documentation/publish/signing-and-distribution-overview/).
Bug 1720923 Comment 8 Edit History
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As Alex did also mention in comment 5, to successfully install non-temporarily an unsigned extension the user has to make sure that an addon id is assigned through the manifest (on the contrary an addon id is generated automatically when the extension is installed temporarily, if one is not explicitly set in the manifest.json file). I'm pretty sure that the issue reporter is very likely related to that (trying to install non-temporarily an addon without an explicit addon id in the manifest.json file). I think it would be reasonable to explicitly mention that the addon id has to be specified in the manifest to successfully install an unsigned extensions in the doc page linked in comment 0 (https://extensionworkshop.com/documentation/publish/signing-and-distribution-overview/), and it may also link the related doc page at https://extensionworkshop.com/documentation/develop/extensions-and-the-add-on-id.