This is certainly a relevant topic when thinking about defending your home in times of civil unrest and riots. I've thought about this a number of times, and it's one of the harder threats to defend against. To effectively discuss this I think we need to break the threat down into at least two categories based on a combination of motivation and execution:
1) The targeted attack. This is a case where an individual or group is targeting you, your home, and your family directly. If they conduct a coordinated attack to force you from the home with fire, well, this will be an extremely difficult threat to defend against. You're likely to lose this one unless you've designed your entire home to defend against this type of attack, or unless you've got a team of defenders setup and ready to fight against this attack 24/7. Incidentally, defending the windows is one of the most obvious areas to prevent fire from easily being introduced to your home - someone already mentioned the steel roll-up shutters and the idea of window film above. Those can help against thrown Molotov cocktails and the like.
2) The angry mob. This is probably the greater threat that most people would face when it comes to an arson attack. In this case I'm talking about riot situations where people are just randomly burning property and buildings during a period of mass lawlessness. In this instance you probably won't be facing an attack that is specifically designed to overwhelm your defenses, but instead you will be facing an attack of opportunity... if your place is easy to burn down without cost to the attacker, you might get burned down (whether or not your family is inside).
In thinking about this second scenario I can picture a number of things you can do to help your odds of successfully mitigating this threat. For starters, having chemical fire extinguishers readily available will be essential to putting out the accelerant that an attacker might spread through a Molotov cocktail or similar type of improvised fire bomb (water isn't the best suppressant against this type of threat). Also, an advanced awareness of impending mob activity can give you time to harden your property against such an attack. For example, boarding windows would do a lot to prevent the most likely and effective way this attack might be executed: by throwing a device through a glass window into a house filled with flammable materials. Similarly, if you knew that riots were imminent, you could also remove drapery or other materials that might allow a fire to quickly grow and spread within a home (ex: rolling up and storing throw rugs in a basement, etc.)... again, fast containment of an individual source of ignition would be essential to prevailing.
But, most of what I've just discussed involves putting out a fire, or preventing its spread, after it has already started. The better defense is to ensure that no fire is introduced into your home in the first place. And, the ugly truth is that effective deterrence in this case is also likely to involve the willingness and ability to mount an effective and violent defense against such an assault. For starters, you certainly want to create distance between the mob and your home. Also, if you see someone in the process of actively targeting you with a firebomb, you're already in a deadly force encounter.
Although rioters tend to be violent and often indiscriminate in their target selection, history has also shown us that encountering armed resistance tends to scatter most rioters, and break their resolve. You need only look through the news archives to see effective examples of this happening during various newsworthy events: the LA riots in the 1990's, the riots in Kenosha, WI, a year ago, and so on. But, even as a good guy defending a homestead, you're still going to be responsible for every round that you fire in defense of that home... your shots may scatter the mob, but they had better be aimed deliberately, and with regards to what constitutes a legal threat that justifies the use of deadly physical force (remember that many of our modern riots are composed of a mixture of "peaceful protesters" and "violent rioters").
The bottom line is that an arson attack can be difficult to defend against, particularly in a city living environment. But, it's also not impossible to do some things to help tilt the situation in your favor.