inside TB-profile folder , emails are usually by-default combined into a single '''mbox''' file per folder. in windows: "%APPDATA%\Thunderbird\Profiles\<TB-profile>\" that is usally here: "C:\Users\<WindowsUserName>\AppData\Roaming\Thunderbird\Profiles\<TB-profile>\" in macOS: "~/Library/Thunderbird/Profiles/<TB-profile>/" that is usally here: mac default volume : "/Users/<MacOsUserName>/Library/Thunderbird/Profiles/<TB-profile>/" check the addon/extension mentioned in [https://support.mozilla.org/en-US/questions/1317998 this] page & also other user's answers , to make backup/copy. you can also do this way : use TB : create SUB-FOLDERS under your '''"Local Folders"''' , similar to mail-accounts like these: mail-account-name-01-Inbox mail-account-name-01-Sent ... mail-account-name-02-Inbox mail-account-name-02-Sent then COPY emails from mail-account's sub-folders into the corresponding sub-folders under "Local Folders". then backup/COPY the entire TB-profile location '''"Profiles"''' (shown below) folder , into Desktop or where you want to: "%APPDATA%\Thunderbird\'''Profiles\'''" that folder is usally here: "C:\Users\<WindowsUserName>\AppData\Roaming\Thunderbird\'''Profiles\'''" you can use this TB-profile again with another TB, by using a command like this from windows "Command-Prompt": "C:\Program Files\Mozilla Thunderbird\thunderbird.exe" -P then choose the profile-name that has your all emails. if not correct TB-profile , you can exit from TB , run TB again by using the commandline to choose another profile. once you find correct profile . remember profile name . exit TB . start TB with above comamndline again & this time you must select the option "Use this profile as default profile", etc option. after above step, you will not need to use command to start TB , you can ust use its ICON/shortcut to start the TB. more info: [https://support.mozilla.org/en-US/questions/1317619#answer-1375337 mbox file conversion]

The knowledge base / help site is your friend. https://support.mozilla.org/en-US/products/thunderbird Searching that is always a good starting point. https://support.mozilla.org/en-US/kb/profiles-where-thunderbird-stores-user-data also discusses backup