Yes, I was speaking hyperbolically.

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.