Wasn't there some change in Firefox recently that priorities flash over other flash objects that do not have focus?
I swear I thought I read something like this in the release notes, but now going back I can't find anything. I noticed some occasional skips in Pandora today, could be network congestion, but it's never happened before. Then I thought I remembered something got changed with regards to how flash objects are handled when not in focus or something.
I cannot tell what version of Flash that you have, it is possible to clear the Flash cache by right clicking on an embedded Flash player and selecting "Settings". In the cache section, clear the storage.
However, I am not sure what you mean by Flash objects.