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seltype
Type: one of the values below
Used to indicate whether multiple selection is allowed.
single
Only one row may be selected at a time. (Default in listbox and richlistbox.)
multiple
Multiple rows may be selected at once. (Default in tree.)

For trees, you can also use the following values:

cell
Individual cells can be selected
text
Rows are selected; however, the selection highlight appears only over the text of the primary column.

For richlistbox, this is new in Firefox 3.5.