You can use this option by the following steps:
- download MMP1.0.00b1 and install it
- find out IDs of XUL elements you want to collapse by DOM Inspector etc.
- open Config Editor
- create new string pref: maxmsgpane.usr_idlist
- set the IDs to its value (multiple IDs are set by using "," as separator)
- restart Thunderbird
This option is experimental. It may be removed in future release.