The device was found in a hair-care product in the Helston branch, after a telephone warning. It was blown up by the bomb squad. On Tuesday firebombs were found at other Cornwall branches in Bodmin and Liskeard.
Earlier yesterday, streets were evacuated while a suspect package was blown up at Boots in Penzance. Branches in Manchester, Derby, Nottingham and Bristol were searched after warnings that incendiary devices had been planted.
No one has claimed responsibility for the attacks, but an Animal Liberation Front spokesman said a group called the Justice Department issued a warning that up to six devices would be planted.