A "grey screen", where the Flash game is supposed to load, is often due to Flash not being enabled within your browser, and for Neopets, itself. Some maps have been converted to HTML5, but "customization" still uses Flash. Firefox, now, periodically asks to enable Flash. Meaning, if it asks, and you play on Neo for a while, going to the maps, it may ask you to enable it, again. Do you receive the prompts to enable Flash, when logging into Neo, for the first time of the day, when going to the "Maps of Neopia"? Latest Flash version is 32.0.0.371. As @RonJB mentioned, is it up-to-date? You mentioned Avast. Does Avast ad a plugin/extension to Firefox? If so, it is possible Avast could be blocking Flash, as most security products consider Flash a security risk. Avast may allow exceptions, too. If so, try adding Flash, and see if it helps.