"There is no excuse for Rossi's reaction and it represents a complete disgrace to everyone making a living from football," the FSS said on its website on Thursday.
"We do not approve of Ljajic's gesture either because players always have to accept a coach's decision and he should have done so in a more professional manner."
Fiorentina sacked Rossi on Thursday and named the club's administrative manager Vincenzo Guerini, whose previous role included duties such as organising kits and buses, as his replacement.
The FSS applauded Fiorentina's decision to dismiss Rossi as coach.
"Fiorentina took the right course of action by relieving Rossi of his post because what he did is completely alien to sport and the fair-play code," said the Serbian FA.
"We wish to maintain our sound relations with Fiorentina but most of all we would like to extend our unreserved support to Ljajic in what must be a very difficult moment in his career."
Ljajic's career seems to be stuttering, with the 20-year-old failing to live up to the potential which prompted his 2010 move to Serie A from Serbian champions Partizan Belgrade.
