In my observation, it really does not matter what browser or email client you utilize. Unless both you and the recipient of the email are encrypting the contents of the email in a unique manner, the email contents are retained in clear text at both of your internet service providers, and it can be captured in realtime at every point of the email's movement from sender to recipient.
In other words, unless you protect the contents of online communications, everything you do on the internet is publicly accessible.