Since Firefox 43, it won’t allow you to install unsigned extensions.
While it’s a good thing, you can revert it in case with the following steps:
To disable signature checks, you will need to set the xpinstall.signatures.required preference to “false”.
- type about:config into the URL bar in Firefox
- in the Search box type xpinstall.signatures.required
- double-click the preference, or right-click and selected “Toggle”, to set it to false.
It will eventually show your unsigned extension with a warning but it should work out fine.