In general yes, but the Firefox example doesn't really apply. Addons are signed with a key Firefox controls, which is something you can get around if you want by using the developer version. In general, addon signing is beneficial. I don't remember exactly how Mozilla screwed up, I wanna say they allowed a domain to expire or failed to update the key/cert when it expired. Outside of conspiracy theories, why you put quotes around accidentally? It was a hassle I'm sure they'd have rather avoided.
Also, Firefox is open source, you could patch it to have another behavior if you want. In other words, you have as much control as you choose to take.