By 'labels' do you mean 'Tags' ?

If yes, then the list of 'Tags' is in the profile name folder in the 'prefs.js' file.

On computer which has list of 20 tags: In Thunderbird

• Help > Troubleshooting Information
• Under 'Application Basics' section - half way is 'Profile Folder' - click on 'Open Folder' button

• A new window opens showing the contents of you 'profile name' folder.
• Scroll down and look for the 'prefs.js' file
• Open the 'prefs.js' file using Notepad.
• Scroll down and look for where you see the first line starting with:
• user_pref("mailnews.tags.
• Highlight all the lines that start with that preference then right click on the highlighted lines and select 'Copy'
• Then close the 'prefs.js' file. No need to save anything because nothing has changed.

• Open a new Notepad file, right click and select paste.
• Now you should have a complete list.
• Save the file as filename 'Tags' to an external drive so you can use it to move to new computer.

On new/other computer:

In Thunderbird

• Help > Troubleshooting Information
• Under 'Application Basics' section - half way is 'Profile Folder' - click on 'Open Folder' button
• A new window opens showing the contents of you 'profile name' folder.

• Exit Thunderbird now - this is important

• Scroll down and look for the 'prefs.js' file
• Suggest you make a copy of that 'prefs.js' file and save to desktop as a backup - just in case you make an error.
• Open the 'prefs.js' file using Notepad.
• Scroll down and look for where you see the first line starting with:
• user_pref("mailnews.tags.

• Access the saved 'Tags' file.
• Copy all the lines: user_pref("mailnews.tags.

• In the opened 'prefs.js' file look for the lines that start with user_pref("mailnews.tags.
• You need to delete them and then in the same space paste in the copied lines, (or highlight all of them and use right click and Paste to overwrite) so you now see a complete list of all those lines.
• There shoud not be any empty lines.
• Save the file.

Start Thunderbird.

Hello Toad-Hall, thanks for the solution, it works!