It means that the person does not know the answer to the question but is interested in the answer. It also means that everyone has gotten too lazy to type in following.
Σ(  ̄□ ̄||) I thought it was like RIP bc no one knows what manga their talking about Isa142
Nah you're thinking of just "f", as in "pay f to pay respects." ff just means they're following because they're also curious on the answer and are hoping someone else will know the answer and they will receive a notification.
What the heck does ff mean I am so confused (๑•ㅂ•)و✧