Under your Firewall -> Rules -> [Interface] are you choosing a gateway on the rule?
Also under Firewall -> Settings -> Advanced, did you "Disable reply-to" ?
Take a search through the forums for the implications of how this works. This may fix your problem.5SF
Since you can not reasonably use NAT on IPv6, you must make sure that not only the firewall routes over the correct uplink, but also that the client devices use the right source address.So I'm afraid the best you can do is policy route on the firewall so the right source address is routed over the right uplink, you will not be able to determine which uplink to use beyond that, since the client makes that decision.
You will have to use one firewall rule for each of the uplinks with the appropriate source prefix and then configure the right gateway in the firewall rule.If those prefixes are dynamic, right now you're out of luck.