prompts.tab_modal.enabled = false has no effect in Firefox 51

[Jim]

I have Ubuntu 16.04 & mint 18.1 installed. In Ubuntu it has the desired 
effect but not in Mint. I think the difference is because in Ubuntu it 
was set to false in Firefox 50 and then upgraded to 51, but in Mint it 
was set to true in Firefox 50 and was upgraded to 51 in that state.

Google has been no help and trying a new profile does not work either. 
Is anyone else seeing this and maybe have a work around?

Regards,  Jim