If the prior profile is still intact, just ignore the prompt for new email, click the 'home' tab and then rightclick near top and activate the 'menu bar' and then click help>moretroubleshootinginformation, scroll down to Profiles and click 'about:profiles' - and your profile should be visible.

A bit of additional info for others in a similar situation. Following David's instructions I was unable to see my old profile in TB despite seeing it in the same folder in file manager. I then created a new profile from "help>moretroubleshootinginformation" "Profiles" and "about:profiles", and noted the profile name created by TB. This new profile is essentially "blank". I closed TB and copied all files/folders from my old profile into the newly created one overwriting the only files there. When I opened TB it went to the "new" profile as default and all my old accounts & mails are present. A bit round about but so far all seems to function correctly so I don't argue w success.