Using Firefox 52.2.1 ESR (for XP-era Windows), Simple Currency Converter worked well until Wednesday (August 2). Suddenly, but after no update of any kind, SCC began to substitute the phrase "undefined 000" in place of any stated amount in dollars.
This utility is a pleasure to use, when it works. Unfortunately, I had a similar interruption of SCC during its first use in 2016, and simply removed the extension. After giving it another chance, it is down again, after perhaps only a week. Since it was working only two days ago, the author may have "broken" his code in certain respects, which affects (at least) Firefox and XP SP3, 32bit.