There is no mobile version of the Thunderbird email client yet. Thunderbird is is a email client you use to send and receive email on your Windows, macOS, or Linux system as there is no Thunderbird Account nor is it some form of service or server. You can access your email in a web browser at its webmail site usually what is after the @ like gmail.com for example or you can setup to send/receive email in a email client made for iOS/iPadOS.