This highlights a security problem: if addons can affect/patch each other, how can you ensure the integrity of the browser?
Example: a malicious addon is released, and it takes some time before the malicious behaviour is discovered, and people delete the addon. But has it injected malicious code into other addons on the system? Now you have to remove all addons to be sure.
Is this outlandish or possible? Has Mozilla implemented any security against such an attack?