Naturally you’re worried about the security of your 23andMe account, and the idea of an extension that talks to 23andMe concerns you – it should!
Rest assured that this extension runs completely within your browser and never talks to any site other than 23andMe.com – this will be confirmed when you install the extension. Anybody suitably competent can review the source code for this extension and confirm that it isn’t doing anything nasty.
You should always be very, very careful downloading an extension for a service like 23andMe – it would be very easy for an extension to copy your data or do things using your account that you’d rather it didn’t. With that in mind, please make sure you never download this extension from anywhere other than this site.