But the group called it a "disturbing coincidence", noting that the Paris branch, headquartered in a working-class district in the northeast of the city, had recently received a phone message calling the activists "witches" who should "burn".
The fire broke out at 5am local time on Sunday in a room where Ukrainian activist Inna Shevchenko, who was recently granted asylum in France, has been sheltering.
She was not present at the time, though several members of the group were asleep in the offices located on the building's second and top floors.
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Among them were Pauline Hillier and Marguerite Stern, who were detained for nearly a month in Tunisia after a topless protest on May 29.
Firefighters said no one was injured.
But Hillier said the group was "shocked" by the incident and "would like to know what really happened".
She said: "We get death threats every day. Yesterday we received a message that said 'burn witches'.
"We find that there are many coincidences and this has happened just one week after the stamp controversy," she said.
Shevchenko was said to have partly inspired a new stamp released on Bastille Day, France's national day, bearing the image of Marianne, the female symbol of the French Republic.
That sparked fierce controversy, with critics of Femen denouncing the new Marianne as a "Christianophobe, hater and ideologue".
Earlier this month Shevchenko provoked Muslim ire when she tweeted at the beginning of Ramadan: "What could be more stupid than Ramadan? What is uglier than this religion?"
