Browser caching and Timthumb

[Kuujiryo]

I've noticed something in Firefox whilst browsing my blog. When I make a new post and create the thumbnail, it doesn't show up immediately for me. Instead, the title of the post is there instead of the thumbnail. It appears there if I use IE, but to see it in Firefox I need to clear my cache, or wait quite a while for it to show up.
I doubt it's a problem specific to Firefox, and I assume this is a problem other people are going to come across as well while viewing my blog. Does anyone know of a way to make it so they won't have to clear the cache to view the thumbnails each time a new post is made?