Oh, don't worry! Having my experience playing D&D when all actual rules were more ... guidelines, I have no problem ignoring or changing things that don't work.
And if a fireball can't ignite toilet paper, that's a rule that doesn't work (IMO).
Now, I assume the intent of the ruling was to avoid the 1e situation of "get hit by a fireball, all your stuff has item saving throws and you lose a bunch of it." Which is fine. I have no problem ignoring that in 5e.
But I cannot suspend my disbelief so hard that tissues become magically immune to fireball.