Hello! Sorry it's taken me so long to get back to this. I've updated Back When to use the latest Ren'Py engine release, since I've heard this error was fixed in later versions. I've never encountered it myself on Firefox so unfortunately I can't test this myself. If you're still interested in Back When and this update hasn't fixed the issue please let me know and tell me what browser you're using, I promise I'll respond sooner this time!