Ultrahigh continuous-wave intensities (>300 GW/cm^2) in high-NA optical cavities enable applications from phase-contrast electron microscopy to ultradeep dipole traps for molecules. However, the intensity can be limited by the optomechanical parametric instability (PI), where mirror vibrations scatter light from one cavity mode into another. We observe PI in a table-top Fabry-Pérot cavity, show that the mechanical modes are MHz-frequency bulk acoustic modes inside the mirrors, and measure their Q factor. By using low-Q mirrors, we achieve >500 GW/cm^2 intensities in an open, free-space cavity.