How can I get rid of these squares? The particle velocity is high (like fireworks). Subdivision of smoke 300, upres of noise 5. Only space is higher. I render specially on 8 samples (engine and volumetry) to create a flickering effect (emitter appears-disappears-appears-disappears)
The squares appear due to the way that Blender renders fluid/smoke simulations. If you want your simulation to have a higher resolution (ie. everything looks smoother), then you can increase the resolution divisions of your fluid simulation domain object at Physics Properties > Fluid > Resolution Divisions.
This should give your smoke simulation a higher resolution and reduce the "squareness" of the smoke trails; however, please note that increasing the resolution of these types of simulations can drastically impact performance, so make adjustments conservatively.