how to get to my thunderbird account on a new computer

If you previously used an IMAP mail account in Thunderbird, then you were remotely viewing emails stored on the server. So download Thunderbird, install and create the IMAP mail account to connect to the server. If you had a POP mail account, those emails would be stored on the computer and depending upon your settings, they may have left copies on the server. Accessing webmail account would tell you if those emails were left as copies on the server. a Pop mail account can be created and download anything currently on the server. So what type of mail account did you use? Did you create a backup recently and store it externally from computer? If you can access the old computer hard drive as an external harddrive, you could locate your Profile folders and perform a backup to use on new computer. I'll supply more info if this is an option. Can you get access to the hard drive?