Mozilla signs extensions as a defense against malicious software. If an extension's files are altered, the signature becomes invalid (i.e., "corrupt"), so Firefox will refuse to load it.

You can disable signature verification by setting xpinstall.signatures.required in about:config to false in Firefox nightly, developer, and enterprise (esr). The setting exists for all versions of Firefox, but it has no effect in released and beta. (Nightly and developer are pre-release. Beta is pre-release, too, but it's supposed to behave as if it's released.) The setting might also work for some clones, but that depends on the clone.

See How can I disable signature checking for Firefox add-ons?.