From MozillaZine Knowledge Base
- This article was written for Thunderbird but also applies to Mozilla Suite / SeaMonkey (though some menu sequences may differ).
Thunderbird has several new features which are enabled by default that increase the size of a profile. If you disable offline folders, global search, and the disk cache Thunderbird will only download headers for messages in remote folders (IMAP account). It will fetch the contents as needed when you open a message in a remote folder, but it will only store it in memory.
This configuration is also useful if you have a 3G mobile connection and need to minimize bandwidth usage. If you sometimes want to use some of the disabled features when using a broadband connection consider using two profiles which use common directories outside of the profile to store the messages. One profile would disable features as described below. The other could keep them enabled. That way depending upon which Thunderbird shortcut you use you can easily switch configurations with minimal side effects. See Profile manager (create a new profile), Moving your mail storage location and Shortcut to a specific profile if you want to do that.
- If you have a IMAP account offline folders are enabled that automatically download copies of the messages in all of your folders. You can disable this feature by unchecking Tools -> Account Settings -> Account Name -> Synchronization & Storage -> "Keep messages for this account on the computer". However, this doesn't delete any existing offline folders. If you have some look at the local directory setting at the bottom of Tools -> Account Settings -> Account Name -> Server Settings to see where Thunderbird stores the files for that account. Exit Thunderbird and delete any files in that directory (and its children) that have the name of a folder but no file extension (mbox files). For example, "inbox." for the inbox folder. Leave the *.msf files alone. The *.sbd files are renamed directories.
The "All Mail" folder in a Gmail IMAP account has a copy of all messages for that account, doubling the number of messages downloaded for offline folders. Thunderbird tries to download only one copy of a message from a Gmail IMAP account and have the folders point to that copy. However, that doesn't help if the message was created using Thunderbird.  If you decide to keep offline folders enabled and have a Gmail IMAP account, uncheck "All Mail" in Tools -> Account Settings -> Account Name -> Synchronization & Storage -> Advanced. As a precaution right click on the Gmail account name in the folder pane, select subscribe in the context menu, expand the folder listing and verify the All Mail folder is not subscribed. Disabling it from being synced should have unsubscribed it. Exit Thunderbird and delete "All Mail." and "All Mail.msf" in the accounts local directory.
An alternative way to remove the All Mail folder would be to login into Gmail webmail using a browser, left click on the gear icon in the upper right corner and select Settings, select the Labels tab, find All Mail, click on Hide and uncheck "Show in IMAP". Logout and delete "All Mail." and "All Mail.msf" in the Gmail accounts local directory in the Thunderbird profile.
Do NOT try to delete the All Mail folder by deleting its contents. That will delete all of the messages for the account when Thunderbird syncs the folder.
- Global searching/indexing uses global-messages-db.sqlite as a pre-built search index. It contains a copy of all of your messages. The search index improves search performance because Thunderbird doesn't have to fetch or parse those messages before searching them. If you have a IMAP account and disable offline folders it will use a background process to download copies of the messages in all of your folders in order to update the search index. Otherwise it copies them from the mbox files. You can disable this feature using Tools -> Options -> Advanced -> General -> Enable Global Search and Indexer. If you do that exit Thunderbird and delete the global-messages-db.sqlite and global-messages-db.journal files in the profile. Help -> Troubleshooting Information -> "Open Containing Folder" will display the profile directory using Windows Explorer (or your file manager).
You can still search messages without global search, you're just limited to searching for messages within the current folder using either Edit -> Find -> Search Messages (Ctrl+Shift+F) or the Quick Filter Bar (enable it using View -> Toolbars -> Quick Filter Bar). You can also create a virtual folder using Saved Search that can contain the contents of any folders in any account. It just won't be as quick as Global Search because it doesn't have a pre-built search index.
- The disk cache mainly caches remote images and SSL certificates. You can disable it by setting the cache size to zero in Tools -> Options -> Advanced -> Network & Disk Space -> Disk Space. If you do that exit Thunderbird and delete the contents of the cache directory in the profile.
- Configure Thunderbird to automatically compact more often. Currently Tools -> Options -> Advanced -> Network & Disk Space -> Disk Space -> "Compact folders when it will save over X MB" is set to 20MB. Lower it to 1MB.
- If you are using a IMAP account set mail.imap.expunge_option to 2. That will automatically expunge (physically remove) any deleted messages (in IMAP accounts) when there are 20 messages waiting to be expunged. Expunging physically removes messages that had been marked deleted and hidden from view, but it works differently than compacting. For example, it doesn't care how much space the deleted messages take. Its also a good idea to check Tools -> Account Settings -> Account Name -> Server Settings -> Clean up ("Expunge") Inbox on Exit and "Empty Trash on Exit". If you do that it will empty the trash and compact both the inbox and trash folder when you exit Thunderbird.
If you are going to create a lot of accounts you might want to modify some default settings so that you don't have to manually re-configure each account. Changing the default settings has no effect on existing accounts, it only effects accounts that you create after modifying those settings. You can change the settings using the config editor at Tools -> Options -> Advanced -> General.
- Set mail.server.default.autosync_offline_stores and mail.server.default.offline_download false to disable offline folders.
- Set mail.identity.default.reply_on_top to 0 to reply below the quoted text, 1 to reply above the quoted text.
- Set mail.identity.default.compose_html to false if you want to compose plain text messages .
Enter mail.server.default or mail.identity.default in the filter: field in the config editor to find other default settings you might want to set.