The actors, who both won BAFTA awards for their roles in The King’s Speech earlier this week, are the latest celebrities to back the “yes to AV” campaign.
Firth, who campaigned with Deputy Prime Minister Nick Clegg ahead of the election, but has since withdrawn his support, says: “The referendum is a once in a generation opportunity to change our clapped out politics for good. I’ll be voting yes.”
They join Stephen Fry, Joanna Lumley and John Cleese, as well as left-wing supporters Billy Bragg, Eddie Izzard, John O’Farrell, Tony Robinson and Richard Wilson, who also support the campaign.
Opposition to the AV system of voting is mounting as ministers battle to get the bill through Parliament.
The “No to AV” campaign group is today expected to highlight that changing the electoral system to AV will cost £250m.
The referendum on the alternative vote system is set for 5 May.
The Lib-Dem backed initiative is part of the coalition Government’s agreement on coming into power.