| Name | Description |
| .autoreg | Temporary empty file that signals a change in the installed extensions. |
| .parentlock (Mac OS X) | See parent.lock |
| abook.mab | Personal Address Book |
| addons.sqlite | Used by add-ons manager. |
| blocklist.xml | Added list of dangerous extensions. |
| cache.sqlite | Used by the Lightning add-on if the "Cache" option is enabled in the calendar properties, to provide offline support. [1] |
| cert8.db | Security certificates |
| cert_override.txt | This is an optional file used to store a security exception. It appears to store the host name , thus preventing you from creating a security exception for a rotating SMTP server. |
| chromeappsstore.sqlite | Unknown. There is a similar file in Firefox profiles used by localstorage/DOMStorage to store about: pages. [2] |
| compatibility.ini | Stores the version and path of the application this profile was last used with. When the profile is loaded with an application with a different version/path, the XPCOM components registration process is triggered. Thunderbird 3.x appears to use the version information to decide whether to display additional tabs such as the migration assistant and whats new. This file is automatically generated and can safely be deleted. |
| components.ini | Lists extension folders with XPCOM components. Replaced with extensions.ini in Thunderbird 1.5 and above. |
| compreg.dat | Lists registered XPCOM components. Automatically regenerated whenever XPCOM registration process is triggered. |
custom-strings.txt in chrome
(does not exist by default) | It can be used to replace text in the user interface. [3] |
| cookies.txt | Obsolete, no longer used. Cookies RSS cookies - Thunderbird |
| cookies.sqlite | Stores all cookies. |
| cookies.sqlite-journal | Journal file for cookies.sqlite |
| cookies.sqlite-shm , cookies.sqlite-wal | write-ahead log files for cookies.sqlite |
| directoryTree.json | |
| downloads.rdf | Download history. Can be deleted to resolve slow downloads or program hangs (Bug 159107) |
| downloads.sqlite | |
| extensions.cache | Lists installed extensions, their IDs, and the folders they are installed to, along with their last-modify time. It is used, for example, to register extensions installed by dropping a folder into one of known install locations. This file is automatically regenerated and can be deleted to resolve various issues. |
| extensions.ini | Lists folders of installed extensions and themes. The file is automatically generated by nsExtensionManager and is used by low-level code to detect chrome packages and XPCOM components provided by installed addons. It can be safely deleted to resolve various issues. |
| extensions.log | Records some errors reported by the Extension Manager. |
| extensions.rdf | Installed extension information. It can be deleted to remove "ghost" entries from the extension list [4] and to resolve various other issues. |
| extensions.sqlite | Stores data about installed extensions, not clear yet if it replaces or supplements the extensions.rdf file. |
| feeds.rdf | Stores your RSS subscriptions. Its in in the Mail\feeds directory |
| feeditems.rdf | Records RSS feeds. Its in in the Mail\feeds directory |
| filterlog.html | Message filter log. If filter logging is enabled its stored in the accounts directory. |
| foldertree.json | Used by the folder pane. |
| formhistory.sqlite | |
| global-messages-db.journal | rollback transaction journal for global-messages-db.sqlite (temporary file) |
| global-messages-db.sqlite | An index of all of your messages created by the global search engine (gloda). Can be deleted if db is corrupt or search isn't working correctly - it will be rebuilt. |
| history.mab | Collected addresses address book |
| hostinfo.dat in News | Contains all the newsgroups available for the current server |
| junklog.html | Junk mail log for that account |
| key3.db | Key database |
| localstore.rdf | Toolbar and window layout and size/position settings. It can be safely deleted to resolve various issues. |
| localstore-safe.rdf | Special version of localstore.rdf used in Safe Mode to set toolbars and window customizations to defaults. [5] |
| local.sqlite | Internal calendar data (in SQLite format) if you use the Lightning extension. |
| lock (Linux) | See parent.lock |
| mailviews.dat | Defines your current message view (you can customize it) |
| menuedit.rdf | Used by Menu Editor extension |
| mimeTypes.rdf | Action to perform when downloading certain types of files. Can be deleted to reset download actions. |
| msgFilterRules.dat in ImapMail, Mail, News | Message filters |
| nsmail.eml, nsmail.tmp, nsmail.html | Temporary files created when sending a message. |
| nstmp | Thunderbird creates a nstmp temporary file when compacting a folder. You should not see it (or nstmp-1, nstmp-2 etc.) unless Thunderbird was interrupted while compacting. |
| panacea.dat | Mail folder cache. It can be safely deleted to resolve various issues. |
| parent.lock (Windows) | Marker showing that the current profile is in use. Can be deleted to unlock the profile. |
| persdict.dat | Personal spelling dictionary |
| people.sqlite | |
| permissions.sqlite | Used by Lightning extension |
| places.sqlite | |
| places.sqlite-shm, places.sqlite.wal | Write-ahead log files for places.sqlite. You should not have a places.sqlite-shm and a places.sqlite.wal directory (instead of files with that name). If you do that causes Thunderbird to be extremely slow. Its not clear what creates them, but the workaround is to delete those two directories. [6] |
| popstate.dat in Mail | Keeps track of which messages have been left on the POP3 server |
| prefs.js | All preferences, including account settings and mail folder locations. See: Modify Thunderbird settings |
| pubring.gpg | Public keys (yours and any you collected) for the Enigmail (OpenPGP) extension. |
| rulesbackup.dat | If msgFilterRules.dat can't be read it backs it up in that file and creates a new msgFilterRules.dat. |
| search.json | Not clear what it is since there is no search.sqlite to cache. [7] |
| secmod.db | Security module database |
| secring.gpg | Private keys for the Enigmail (OpenPGP) extension. |
| session.json | Default windows layout. Thunderbird 3.x deletes this file after reading the settings. Its re-created and the layout saved when Thunderbird 3.x terminates normally. |
| Signons.sqlite | Encrypted saved passwords, requires key3.db to work. It used to use signons.txt or signons3.txt. |
| storage.sdb | Internal calendar data (in SQLite format) if you use the Lightning extension. Its been replaced by local.sqlite but is still maintained for compatibility in case you switch between different Thunderbird+Lightning versions. |
| training.dat | Custom training for Junk Mail Controls |
| traits.dat | Bayes traits training data used by the TaQuilla add-on |
| tmprules.dat | Temporary file used when modifying message filters. |
| Unsent Messages. | The "Unsent Messages." mbox file in ..\Mail\Local Folders is displayed as the Outbox folder by recent versions of Thunderbird. That folder is used to queue unsent messages if you use "Send Later". |
| urlclassifier2.sqlite | Contains information on known phishing sites. Obsolete. |
| urlclassifier3.sqlite | Contains information on known phishing sites. Seems to be a replacement for urlclassifier2.sqlite, which replaced urlclassifier.sqlite. |
user.js
(does not exist by default) | User-set, overrides the preferences in prefs.js |
userChrome.css in chrome
(does not exist by default) | User-set CSS to change the way the application looks |
userContent.css in chrome
(does not exist by default) | User-set CSS to change the way webpages look |
userChrome.js in chrome
(does not exist by default) | User-set JavaScript to change the way the application works |
| virtualfolders.dat | Saved Search folder settings |
| webappsstore.sqlite | |
| xpti.dat | Lists registered XPCOM interfaces. Automatically regenerated whenever XPCOM registration process is triggered. |
| XPC.mfl | A cache file for components (XPC) |
| xul.mfasl (Linux) | See XUL.mfl file |
| XUL.mfl (Windows) | Cached user interface data. Can be deleted to resolve various issues.
File location changed in Thunderbird 1.5. [8] |
| XUL FastLoad File (Mac OS X) | See XUL.mfl |
