Deleting messages in Mozilla Thunderbird doesn’t actually delete them completely, it marks them as deleted and hides them from view. This can speed up the process, but these messages still take up space on your hard drive. Compacting is the process of completely removing these marked messages, freeing up space in your inbox. Here’s how to compact folders in Mozilla Thunderbird, both manually and automatically via settings.

Select File > Compact Folders. If your folders are large and you have deleted a lot of messages since you last compacted them, this process may take some time.

You can set Mozilla Thunderbird to automatically free up disk space, with or without prompting. Here's how:

Select Thunderbird > Preferences.