@droftarts Ok so this is strange, Firefox appears to be working now.
The only thing I think that has changed is I swapped the service order of WiFi and Ethernet (to put WiFi at the top and Ethernet directly underneath) in MacOS System settings > Network > Set Service Order.
I am using an Ethernet Mini 5+ BTW.
In any case, everything is now working as expected!
Edit: Come to think of it I have experienced similar behaviour previously with Firefox. It really doesn't like when the IP address? changes. It kinda gets stuck and you have to restart it.