Yes, it's definitely possible and have been done. However what kind of ISP line are we looking at? Lease Line, Cable, DSL? Most important, they must be running with fix Public IP, and the webserver should be assigned with an IP address for each ISP.
However, it is not possible to just use your router and your firewall, unless your firewall allowed link load balancing.
Reason is router, firewall and servers usually default all packets to a single default route. With two links, you will have two routes, and the routing can be screwed if the packet comes for the ISP segment which is not you default. Router can handle routing more then one default route, however you will need to apply for ASN numbering for your IP address. Some newer routers does cater for link load balancing. Some firewall like StoneGate have link load balancing ability, which can cater for two link routing.
You might need additional equipment or software to achieve what you need. But it is definitely possible.