Complex sentence: Bric-à-brac was used as ornament on mantelpieces, tables, and shelves, or displayed in curio cabinets--sometimes these cabinets have glass doors to display the items within while protecting them from dust.
Split into simpler sentences: Bric-à-brac was used as ornament on mantelpieces, tables, and shelves, or displayed in curio cabinets. Sometimes these cabinets had glass doors, to display the items within while protecting them from dust.