If you just downloaded FireFox (not Foxfire), the you have the most current version and it should work just fine. I have never seen an error message telling you that your "browser is not strong enough". Browsers do not need to be strong, Your bandwidth needs to be strong enough to move data up or downstream.
Have you submitted a support ticket? Support tech will be able to remotely test your system, which us Community members are not able to do. Click Help, then click Report a problem.