If you were there

[Jamaica Rose]

Lass,

The ability to easily start a fire would be very useful.

However, instead of a lighter, flint and steel, with some accompanying tinder, was very effective in starting fires (once you knew the knack of using it), and easily accessible in the 17th century. So you shouldn't waste your "one modern item" on a lighter when there are many other things already proposed that would be much more useful.

Blackheartedly yrs,

--Jamaica Rose

[the Royaliste]

and a flintkock'll start a fire and fire a cannon