Andrew Hall's century was the performance of the day, the South African all-rounder making 113 in his side's 355 all out, while Rob Newton (81) and David Willey (77) offered willing support.
Ollie Freckingham chipped away to end up with a career-best return of six for 125. The Foxes then reached 89 for one at a good rate before the close, Greg Smith with 36no.
England limited-overs batsman Ravi Bopara showed his class with a knock of 145 to hand Essex the lead over Glamorgan at the SWALEC Stadium.
His 265-ball stay saw Essex to 343 for nine, replying to the hosts' 231 all out.
Australian seamer Michael Hogan ended the day with five for 68.
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