28 pages, 7 captioned figures, 2 tables, authors from page 23, submitted to PLB, figures at http://aliceinfo.cern.ch/ArtSubmission/node/3162 ; see paper for full list of authors ; Electrons from heavy-flavour hadron decays (charm and beauty) were measured with the ALICE detector in Pb-Pb collisions at a centre-of-mass of energy $\sqrt{s_{\rm NN}}=2.76$ TeV. The transverse momentum ($p_{\rm T}$) differential production yields at mid-rapidity were used to calculate the nuclear modification factor $R_{\rm AA}$ in the interval $3