Complex sentence: In this, Mason opposed the interest of the family of Thomas, Lord Fairfax, who wanted Fairfax and other existing counties expanded instead, which may have contributed to Mason's decision not to seek re-election in 1761.
Split into simpler sentences: In this, Mason opposed the interest of the family of Thomas, Lord Fairfax, who wanted existing counties expanded instead, including Fairfax. This difference may have contributed to Mason's decision not to seek re-election in 1761.