HTMaiL is a web development extension for Firefox that allows users to insert HTML code into email messages, enhancing their visual appeal and professionalism. This subscription-based tool integrates effortlessly into the webmail compose interface, providing an intuitive HTML editor where users can create, paste, or import their custom HTML code. The extension also features a preview function, allowing users to see how their email will look before sending it, ensuring a polished final product.

One of the key features of HTMaiL is its ability to convert internal and external CSS styles into inline styles, which prevents style leakage during the email sending process. Additionally, it offers an adaptive HTML editor theme that aligns with the operating system's theme. While this extension is powerful for individual email customization, it is important to note that it does not replace mass mailer systems and has limitations regarding CSS Media Query and external fonts.