You can either install it unsigned with Firefox Developer Edition and it will be permanent. Or you can sign it yourself (you don’t need to publish it on AMO): https://extensionworkshop.com/documentation/publish/signing-and-distribution-overview/ and it will work on regular Firefox.

You can build it yourself from source then.

For assets, you would be distributing other people’s work without permission. Some companies scan online content for digital fingerprints of copyrighted material. Think Youtube content ID but they are other tools out there.

As for software you used without a license, the work you did doesn’t matter in that regard, you’re only liable for using unlicensed software by bypassing copyright protection methods. You’re not distributing it. Their DRM (even cracked) might send them enough identifiable information to sue you (in theory).

I think they meant « by ».