Note that the top-level manifest.json is for Chrome. To use it with Firefox, you have to copy "manifest/firefox-manifest.json" onto manifest.json first.

Though be warned, after a restart Firefox doesn't re-enable side loaded extensions that were added in the "debug" mode.

Also, Chrome (Google) has a habit of randomly disabling or removing side loaded extensions. I even had a side loaded extension that Google forced removal of because Google deemed it contained malware (which it didn't, I know because I wrote it).