  The heavy element content ("metallicity") of the Universe is a record of the
total star formation history. Gas-phase metallicity in galaxies, as well as its
evolution with time, is of particular interest as a tracer of accretion and
outflow processes. However, metallicities from the widely-used electron
temperature ($T_e$) method are typically ~2x lower than the values based on the
recombination line method. This "abundance discrepancy factor" (ADF) is well
known and is commonly ascribed to bias due to temperature fluctuations. We
present a measurement of oxygen abundance in the nearby (3.4 Mpc) system, Mrk
71, using a combination of optical and far-IR emission lines to measure and
correct for temperature fluctuation effects. Our far-IR result is inconsistent
($> 2 \sigma$ significance) with the metallicity from recombination lines and
instead indicates little to no bias in the standard $T_e$ method, ruling out
the long-standing hypothesis that the ADF is explained by temperature
fluctuations for this object. Our results provide a framework to accurately
measure metallicity across cosmic history, including with recent data reaching
within the first billion years with JWST and the Atacama Large Millimeter Array
(ALMA).
