Email, Messaging, & Video Calls Email Compacting Folders in Mozilla Thunderbird by Heinz Tschabitscher Writer A former freelance contributor who has reviewed hundreds of email programs and services since 1997. our editorial process Heinz Tschabitscher Updated on July 28, 2019 Email Yahoo! Mail Gmail Tweet Share Email When you delete a message in Mozilla Thunderbird, Netscape or Mozilla, it is not immediately physically removed from the mailbox file but only hidden from display. This speeds up things considerably, but it also wastes valuable hard disk space. Compact Folders Now and Then in Mozilla Thunderbird, Netscape, or Mozilla To reclaim that space for the current account and compact folders in Mozilla Thunderbird, Netscape or Mozilla: Select File | Compact Folders from the menu. If your folders are big and you have deleted lots of (large) messages since the last compacting, this can take some time. Have Mozilla Thunderbird Compact Folders Automatically To set up Mozilla Thunderbird to free disk space automatically (with or without prompting): Select Tools | Options… (Windows), Edit | Preferences (Linux) or Thunderbird | Preferences… (Mac) from the menu in Mozilla Thunderbird.Open the Advanced category.Go to the Network & Disk Space tab.Make sure Compact folders when it will save over ___ MB.Enter the desired disk space threshold. The default value is 20 MB, which should work fine under many circumstances; if you find Mozilla Thunderbird compacting often — because you receive much mail and messages containing attachments in particular —, you can increase the trigger to 100 or 200 MB, for example.Click Close. To compact folder when you are prompted: Click Compact Now under Do you wish to compact all local and offline folders to save disk space.To make Mozilla Thunderbird compact without prompting in the future, make sure Always ask me before compacting folders automatically is not checked. To choose whether you will be prompted when Mozilla Thunderbird is about to compact folders automatically: Select Tools | Options… (Windows), Thunderbird | Preferences… (Mac) or Edit | Preferences (Linux) from the menu in Mozilla Thunderbird.Open the Advanced category.Make sure the General tab is selected.Click Config Editor….Now click I'll be careful, I promise!, if you are prompted.Type "mail.purge.ask" (not including the quotation marks) under Search:.Double-click mail.purge.ask under Preference Name to toggle its setting (visible under Value):false (also set when you uncheck Always ask me before compacting folders automatically when prompted) — do not prompt when automatically compacting folderstrue (the default value) — ask before compacting folders automatically.Close the about:config preferences window.Click Close. Messages Missing After Compacting If, unexpectedly, you find messages missing or deleted emails appearing after compacting your folders in Mozilla Thunderbird, you can try rebuilding their indices.