Firefox have started the death process.
IHG will soon not work with Firefox at all. If you want to carry on using IHG, it's quite simple, use Pale Moon for IHG. YOU CAN HAVE PM AND FF INSTALLED TOGETHER
. You can even have them both open at the same time
IHG is now listed on the PM addons page. There are links elsewhere for the PM home page, or even do a google search for Pale Moon. Short to medium term, it's the solution for most IHG users. Long term - who knows.
Install PM and IHG, just carry on as normal - you won't have to muck about with FF settings in about:config.