I have been using Tbird for years. I currently have 3 email accounts, an icloud and two gmail. One gmail account is personal, the other is a personal account but used by 3 people for a committee we're on. The first two accounts are ok but the committee account has lost most of its messages at gmail, but they still seem to be on my computer.

Despite having used T'bird for a long time I still need detailed instructions in how to proceed to put the messages back onto the gmail account, hopefully without touching the other two accounts