Instead of individually selecting messages to be archived, you can instead use an advanced search to quickly find all messages in various folders and subfolders that meet your search criteria (e.g., older than 90 days), and then move those messages all at once to an archive folder of your choice. Here’s one possible way to do this:

1. Create a new folder in Local Folders and call it "Archive".
2. Go to the "Edit" menu and choose "Find -> Search Messages...".
3. Where it says "Search for messages in", select "choose this folder" for the account whose messages you want to archive. If you are using Thunderbird’s Global Inbox with multiple POP accounts, you can select Local Folders as the account.
4. Make sure that the checkbox for "Search subfolders" is checked.
5. Define the search criteria as "Age in Days is greater than 90" (or however many days you want).
6. Click the "Search" button. The list of "old" messages will appear in the lower pane.
7. Select all messages listed in the search results, and use the "File" button to move them all to the "Archive" folder you created in step 1.
8. If you have more than one mail account that you want to archive, repeat the above steps for each additional account. (If all your accounts are using the Global Inbox, you will not need to repeat the above steps.)

Note:

• After step 6 above, Thunderbird users can also make a Saved Search folder so that in the future this same search (steps 2-6) can be performed with a single click. If you want to exclude certain folders from being searched, just right-click on the Saved Search folder, choose "Properties...", click the "Choose" button, and (de)select folders as desired.

• The above procedure will put all of your archived mail into a single folder, and it thus will not preserve your folder structure.