The error "HTTP 429 Too Many Requests" does not indicate a support issue with a client browser or server protocol.
It generally means that the Wikipedia server received more requests than allowed from your IP address, for a given period (e.g. per minute), for a certain kind of request (e.g. thumbnail requests, or more specifically a certain thumbnail width).
This means Windows XP or Firefox is not relevant to the situation at all, but your Internet connection. Perhaps your internet service provider, or your building/home/office, is sharing the same IP address for a large number of people? (All of whom may be browsing Wikipedia, or perhaps one person has a device with a virus that is trying to attack Wikipedia.) You are running into a rate limit.
This would explain why some JPEGs work, as indeed it is not about the image, but about the Internet connection.
You could confirm by using the same device over a different connection (e.g. try a neighbour WiFi, or someone's mobile hotspot). Or perhaps you are using a busy VPN.
-- Timo