This is likely related to the warning that suchmoon had posted about on the BPIP extension thread, which it seems like firefox is doing for all extensions that are not officially listed on their "recommended" plugins list. Not sure if it is a different warning you are receiving or not, but I'm sure it is likely following the same idea of not being on an official list deemed OK by Firefox.

Good news is, for anyone tech-savvy, extensions are open-source.. so anyone can verify the safety of the code independently, or potentially hire someone to screen it for you.