According to Wikipedia a freedom fighter is a person engaged in a resistance movement against what they believe to be an oppressive and illegitimate government. I could assume this definition could apple to all kinds of people especially activists. Given the progress in humanity, human rights, and discrimination over the last 20 years, I would say there is a plethora of people who have contributed positively to human society.
Yes, for a long time Nelson Mandela has worked as a freedom fighter that has worked his entire life to positively improving the human race. He started a long time ago as a freedom fighter, and up until his recent death stayed in the game to help out our world.
One country's freedom is another person's terrorists, and most of the freedom fighters that have been seen over the past 20 years have been anti western and anti American, and have been very much against freedom. Freedom fighters, as they have been called, have killed many civilians and have blown up innocents.
No, no freedom fighter in the past 20 years as thoroughly contributed positively to human civilization, because most of them just kill innocent people. Freedom fighters are also notorious for destroying other people's property. They claim to be fighting for people's interests, but they use destruction in order to get their point across.
It is hard for one freedom fighter to have an effect on an entire populace the way William Wallace or George Washington did. It simply is too big of a world now. The last freedom fighter, if we could call him that, was Osama bin Laden. He didn't positively contribute to human civilization whatsoever.