In the context of your original point of enquiry, my interpretation of MSER is;- As long as the other container is used for storage of inert materials, and does not contain inflammable substances in quantity (say the farmer wanted to process chip shop oil/diesel for his tractor), then it would be difficult for the local trading standards officer to object to another container sited next to your firework container.
What about pesticides / fertilisers as it has to be associated with a farms business or it will need planning permission anyway. It is certainly a tricky one.
I would not want a container of pyro next to anything I was storing in another container esp. if it has any value. I guess you should think of the worst case scenario and you will probably do the right thing (basically do a risk assessment/hazard study to show due diligence).