My hyperbole also presumes that Gecko continues to be updated, though the browser would get no further updates.
This hyperbolic hypothetical is truly impossible, since Firefox is open-source. It would continue to be maintained by SOMEone.
Six months might be a bit pessimistic. It might start being less reliable within six months, but the pace of WHATWG RFCs has been dwindling gradually since the mid-2000s. Honestly, I think operating system changes would be more likely to render Firefox’s codebase obsolete before web standards do.