Add the same email account ("address") to the mail client in your phone. You'll probably need to ensure that neither Thunderbird nor the mail tool on your phone deletes messages before the other can see them. The best (IMHO) solution is to use IMAP for maximum visibility of all email on both/all your devices. Failing this, if you must use POP, set both Thunderbird and the phone to leave copies on the email server. Some email providers allow only one connection at a time. Some phones set up multiple connections and consume all the connectivity offered by your email server. In both cases, it helps to make sure one device is turned off (or is off-line) when using the other.