how do I know that I have done all I can reasonably done to make my personal and private data secure?

Aloha, I get anxious, sometimes very anxious when I wonder how safe my financial and personal information is online using my phone. We do not have a laptop, tablet, or desk PC, I have an unlimited data plan thru total wireless and it's all i use for internet oh, other than T-Mobile home internet for streaming. Don't use home internet to do anything by scroll and view. I begin to question myself, and that leads me being introduced to security systems and companies that provide VPN and other thing with initials. Reading through the material on the topics of security and privacy, it seems that I have these extra features already included through FIREFOX. But yet, nothing is as simplistic as that. Or is it? Do I have all the special security features just by using FIREFOX as my browser? I hope this isn't the most annoying topic you need to explain, comfort, assure to other people ever. Thanks, if you and just tell me what I need and how to do it for my cell that'd be super groovy awesome. Sincerely and Respectfully, Laura