We introduce the freeze operator in metric first-order temporal logic (MFOTL). It records a relation and provides means to refer to it independently of temporal operators surrounding the reference. As such, it generalizes the (half-order) freeze quantifier available in timed propositional temporal logic to a first-order operator. Semantically, the freeze operator is similar to the non-recursive let operator, but with a subtle yet profound difference. We provide a monitoring algorithm for formulas with freeze operator and implement it in the MonPoly and VeriMon monitors. The VeriMon implementation is accompanied by a correctness proof checked by the Isabelle proof assistant. We evaluate the performance of both implementations and demonstrate how the freeze operator can be used in rewrite rules that aim to bring MFOTL formulas into relational algebra normal form without affecting the monitor’s progress. This yields a strict semantic improvement over previous rewrites, although not always performance-wise.