I used to work there, security was mostly decent with the exception of not encrypting files client side. We advocated for it many times but the founders wanted

I used to work there, security was mostly decent with the exception of not encrypting files client side. We advocated for it many times but the founders wanted to keep the UI as simple as possible for users, which means that forcing users to send the encryption keys to the receiver via a secondary channel (ie not via WeTransfer, which would defeat the purpose as we would be able to MITM it) was a no-go. You can always encrypt the files yourself with the method of your choice and then send the encryption key over Telegram/Signal/whatever. The company has existed for over 10 years now and is profitable, so it's not going to disappear overnight or anything. That said, the thing that apparently killed Mozilla Send (becoming a hub for spam and child abuse material) was much easier to handle. The last big project I was involved in was an automated photoDNA scanner to detect suspected child abuse material. It would pull in various lists of hashes of known bad material and flag any matches for manual verification by the department of the Dutch police that handles such things.