The author switched from Gmail to Thunderbird after 15 years, citing frustrations with Gmail's ad inundation and cluttered features. They appreciate Thunderbird's offline email access, extensive customization options, open-source nature, absence of forced AI features, free distribution without targeted ads, cross-platform compatibility, reliable notifications, effective multi-account management, customizable junk filtering, and its focus solely on email functionality. These aspects have led to a more streamlined and efficient email experience compared to Gmail.