- Brother of Colin Firth and Katie Firth (a voice coach).
- Once shared a flat with Rufus Sewell.
- He and brother Colin Firth have both played a Prince Albert of the British royal family. Jonathan played Albert, the Prince Consort to Queen Victoria, in Victoria et Albert (2001). That character was the great-grandfather and namesake of the subsequent Prince Albert who became King George VI, played by Colin in Le Discours d'un roi (2010).
- Brother-in-law of Livia Giuggioli.
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