'A fortnight at least of leisure and peace of mind, to crown every warmer.'-ref. to warmers in Emma, ch. 16

Foot warmers were the Regency era way to keep warm in carriages whilst travelling in winter.

Most foot warmers were simple wood and tin boxes with metal trays holding the hot coals.
Foot warmers were easily portable and people could carry them to church or inside a carriage. Apparently, women's long skirts hung over the warmer holding the heat about their feet.
Over the C19th, foot warmers were primarily used in carriages.

There is a picture below of a Regency era box footwarmer from Vic's same article, 'Ways to Keep Warm in the Regency Era', part 2.