It is possible to do what you want. The reason it won't work at this point is because firefox is running one profile. You need to set up firefox to run two profiles at the same time and then it will work (or 3 profiles if you were checking 3 yahoo accounts simultaneously and so on...)
Here is how to do that.
Read intructions
Also,
READ THIS from the Mozilla Firefox website about how to manage multiple profiles.
This Google search might help with more info.