hey dear taron, thats strange - the brush behaviour between Opera Browser and Firefox Browser and Mouse/Wacom Pen is very different.
Opera feeling is even, brush tip round, filled, standard feel.
Firefox same brush setting, brush tip ring-shaped, shaky - but with Wacom pen only. If I use the mouse, the brush tip is like in Opera.
But is okay, maybe you can call this effect FirefoxBrush feeling, it creates very different textures etc.