I became the usufructuary of my spouse's property upon his/her death. Do I have to declare the value of freehold assets to calculate the IFI?

No, if the dismemberment of ownership results from the option exercised within the framework of your legal rights (article 757 of the Civil Code), you only declare the value of the usufruct.

This value is determined according to the scale of article 669 of the General Tax Code, according to your age.

On the other hand, if the option is exercised within the framework of a donation to the last survivor (article 1094-1 of the Civil Code), you must declare the goods for their value in full ownership (art. 968 CGI).