To start with, let's explain what would be end-to-end encryption - basically, first, it ensures that the message travels uninterrupted from the sender to the recipient; second, the originating party encrypts the message and ensures that the receiving party has the key to decrypt the message while if the message is intercepted by third party it wouldn't be readable without the right key.
One of the most popular programme for email encryption is PGP (Pretty Good Privacy). It is compatible with Thunderbird and Sea-Monkey email clients. When it comes to email encryption, it is always better to use an email client as web-based email has its weaknesses due to web-browser's vulnerability. I, personally, use Sea-Monkey, it's a web-browser and built in email client. In my opinion Sea-Monkey is one of the most secure web browsers as well as it's very light and good; the other great advantage is its email client.
So here I will go on with some step-by-step instructions as how to start using PGP with Sea-Monkey email client: 1) Install Sea-Monkey from here; 2) Set-up your email; 3) Install PGP Enigmail from here; 4) Generate your private and public key from Engimail button (on top of your email client); 5) Happy secure and encrypted emailing! As you see I am not that good in explanations so in case this guide is not good enough try to google out more on this topic or email me for help.