Super, tausend Dank! Das hier hat grade funktioniert:
Find your profile, that is C:\Users\name\AppData\Roaming\Thunderbird\Profiles\whatever.default on Vista and Windows 7 (the change made shouldn't apply to Windows 2000 and XP)
create a folder named chrome in there and enter it
right-click and select New → Text File and open it
copy-paste the style code:
listitem, treechildren::-moz-tree-row {
min-height: 20px !important;
}
close the text file and rename it to userChrome.css (confirm the warning on changing the file extension)
restart Thunderbird and see if it made a difference