Found the rule that's adding the line break. It's:
.dGSDInboxList .innerList .innerListItem .plain,
.dGSDInboxList .innerList .innerListItem .tableWrapper,
.dGSDList .innerList .innerListItem .plain,
.dGSDList .innerList .innerListItem .tableWrapper {
display: table;
}
If I disable that rule on the .tableWrapper via FF's Inspector tool the line break goes away. However I've not found the right incantation to put in StyleSheet to override it. (Nor do I know what ill effects doing so might have elsewhere....)