I have finally found an edition of the Dragonriders of Pern trilogy - living in Italy, I only had access to it thanks to my father's old books, and he doesn't own the complete series - so I have been immersed in that world and started imagining its less peaceful, alternative version.
The bigger the dragon, the rarer its occurrence in the population. These dragons have, other than their physical weapons, the ability of breathing fire through the digestion in a second stomach of firestone, and of teleporting to a different place, time, or both, as long as they have the mental image of the location at the specific time; or the generic image of the location if the teleportation is through space only. I will ignore here any specific exceptions that occur in the books. Each dragon is impressed by a rider when it hatches, and it lives the rest of its life in a symbiotic relationship with "its human".
The technological level throughout the planet is mostly European medieval, with a few exceptions (Rudimentary flamethrowers as the most notable).
Dragon-folk live in weyrs, communities built in inactive volcanoes most commonly, as you can see here.
Common people live in settlements much like the ones that were usual in the European middle age. Dragonfolk are either descendants of inhabitants of the weyr, or people brought in by "normal" settlements, according to the needs of staff and dragon riders (depends on how many eggs the queen dragons lay).
Now, while in the official Pern, dragonfolk are generally peaceful (with individual exceptions), I can easily imagine a culture like this realizing that they will not need much to grab power. Once the different weyrs have claimed their territory, they will turn against each other to obtain land, goods, privileges...
While the offensive potential of a dragon is immense, how would it be possible to defend yourself from a few hundred dragons that all of sudden appear out your window (figuratively speaking)?
I suppose the relationship among weyrs would be much like the Cold War, with isolated fights in neutral territory, but no large scale offensive, with large use of sentinels and deterrents (you burn my house to the ground, and within 1 second I am burning your house to the ground too).
Common people, on the other hand, will be largely disadvantaged. I expect their costumes to change over time to become similar to what was mentioned in this question. This may not be enough, as these dragons are intelligent, numerous, guided by human riders, and can friggin' teleport.
Which would be the most effective defense against a weyr in full force (assume 300 fighting dragons)? Either by dragonfolk, common people, or an alliance of both.
Feel free to ignore the ability to time travel, as it may be tricky, though fascinating to explore in terms of military strategy. If you want to tackle it, keep in mind that if you will go back in time tomorrow to do something, somewhere, right now, it means that in this moment there is a double of yourself doing whatever, wherever. You cannot perceive a specific time in one way and then go back to alter it, if you could it would be changed already - that is the assumption in the books. You don't change the past, it is already changed, and then the cause will occur.
It's way more physically and mentally demanding to travel through time, and the presence of two versions of yourself within, say, the same city, is enough to make you almost faint. Furthermore the risk of ending somewhen else entirely is high.