To install extension outside the store, it needs to be signed by Mozilla (all addons in store are signed, even after they are removed), and it must be NOT present on the “malware list”. If the addon is removed due to serious policy violation, it will be marked as malware and removed from existing devices.
Your Firefox will not allow you to install addon that’s not signed, even if it’s not coming from store.
Regarding extensions in Chrome store, they are not directly installable to Firefox.