Offline Firefox password manager.
Simple password manager that works offline.
Download most recent release.
Verify the file was downloaded correctly by comparing against the checksum:
$ sha512sum file.xpi
The hash outputted by sha512sum should match the hash from the checksum
Goto about:addons click the gear -> "Install Add-on From File" -> select .xpi file.
Done!
This error is caused by using an old version of Firefox that is known to remove add-ons.
Updating to the newest version of Firefox and reinstalling add-on will fix this.