Selecting a folder will prompt a synchronization with the mail server, but only new messages received since the last syncing are downloaded. If you want a folder to automatically sync when getting new messages, right-click, Properties, and check the box 'When getting new messages...'.

To download headers only, uncheck 'Keep messages ... on this computer' in the Synchronization & Storage section of Tools/Account Settings, or click Advanced to download message bodies for selected folders. When a headers-only message is opened, the message body is downloaded and stored in the cache, but not in the profile folder as when messages are downloaded for offline access.

http://kb.mozillazine.org/Using_Gmail_with_Thunderbird_and_Mozilla_Suite

Thanks for your prompt reply.

Selecting a folder will prompt a synchronization with the mail server, but only new messages received since the last syncing are downloaded. If you want a folder to automatically sync when getting new messages, right-click, Properties, and check the box 'When getting new messages...'.

OK. And what about the first time I configure an account? Do I have to select every folder and subfolder so TB performs a first time synchronization with the server? I'm asking because I normally work with 3 different Gmail accounts and every account has dozens of folders and subfolders. If I don't select every folder, then the search function will never find the contents of message's bodies?

To download headers only, uncheck 'Keep messages ... on this computer' in the Synchronization & Storage section of Tools/Account Settings, or click Advanced to download message bodies for selected folders. When a headers-only message is opened, the message body is downloaded and stored in the cache, but not in the profile folder as when messages are downloaded for offline access. http://kb.mozillazine.org/Using_Gmail_with_Thunderbird_and_Mozilla_Suite

I would prefer to have the body of messages downloaded and stored in the profile folder, so the search function finds them whenever I need to. However, I don't know if the option "Don't download messages larger than" will effectively download bodies or only headers. I need the bodies for universal search, but I don't need the attachments yet (only when I open the message). Please advice.

Thank you in advance.

When you first add an account, do it offline from File/Offline/Work Offline so you can open Advanced config. from the account setup window and uncheck the box 'Keep messages... on this computer'. That way, when you go back online TB will sync with the server and find the folders, but will only download headers. Then, you can choose which folders to subscribe and which ones to download for offline use. It's then possible to search the message bodies of the offline folders from Ctrl+Shift+F.

You'll have to do some tests, but I believe that setting a size limit will download just the headers for messages larger than the limit. The situation without a limit is a bit more complex. See this article for an explanation.

When you first add an account, do it offline from File/Offline/Work Offline so you can open Advanced config. from the account setup window and uncheck the box 'Keep messages... on this computer'. That way, when you go back online TB will sync with the server and find the folders, but will only download headers. Then, you can choose which folders to subscribe and which ones to download for offline use. It's then possible to search the message bodies of the offline folders from Ctrl+Shift+F. You'll have to do some tests, but I believe that setting a size limit will download just the headers for messages larger than the limit. The situation without a limit is a bit more complex. See this article for an explanation.

Thank you sfhowes. It worked by adding the account and then going offline. All messages on all folders were downloaded. It took a while, and after everything was downloaded, I went back online. Then, in order to have all folders synced permanently I enabled the setting mail.server.default.check_all_folders_for_new which worked partially ok, with some label issues that I posted here.