From MozillaZine Knowledge Base

When you receive a message with attachments, the attachments are listed in a separate pane at the bottom of Thunderbird's preview pane or message window. The more attachments you receive, the more space this pane takes up, and by default the attachment pane does not stop growing. As a result, if you receive a large number of attachments the attachment pane can take up so much space that you can't even see the message body. To change this, so that the attachment pane will have a fixed height and use a vertical scrollbar, exit Thunderbird and insert the following into your userChrome.css file:

 /* Attachment pane at bottom of message window */
 #attachmentView {
   -moz-appearance: none !important;
   height: 40px !important;
   overflow: auto !important; }

Other information

• You can change the pane height by modifying the number in 40px . If you reduce the height below 28px it typically clips the bottom of the filename.
• The line "-moz-appearance: none !important;" may not be necessary, depending on the theme you are using. You can experiment with this line in/out to see how it works with the theme you are using.
• For further discussion, see this Mozillazine forums thread.

See also

• Attachments list