We report measurements of the extinction ratio (ER) of white light generated upon irradiation of BK7 glass by ultrashort $\left(36\mathrm{fs}\right)$ laser pulses with incident power $\sim {10}^3$ times larger than the critical power for self-focusing. At low incident powers, the continuum is symmetric about the incident laser wavelength; at high powers it becomes broader and distinctly asymmetric towards the blue side. We observe that ER degrades by 100-fold after the onset of multiphoton-induced free-electron generation (at incident intensity $\sim 2\mathrm{TW}{\mathrm{cm}}^{-2}$ ), which also corresponds to the onset of asymmetry in white-light spectra.