Since we don’t know exactly what is going wrong, nobody can give a 100% sure-fire solution. So let’s explore some possibilities logically and see if anything works, ok? Obviously if Firefox used to work and has stopped, something has changed. The task is to track down what. This will involve asking a lot of questions to poke into corners and see what’s there. Please recheck your answers - you’d be surprised how often people insist they have tried something and when they try again it turns out they had some small detail wrong the previous time!
There have certainly been ongoing changes in the 3DW itself since the release of SU 2017. These could affect you even if (as your profile says) you are using SU 2016 and Firefox. That is why the first recommendation is always to clear the browser cache and cookies to make sure you aren’t getting old versions of any part of the 3DW. It’s also why you should review your Windows internet security settings and also any firewall settings. A revision in how the 3DW loads could potentially be running afoul of some security trap.
Looking at Firefox, have you tried re-downloading a model that you succeeded getting previously (I’m trying to eliminate the possibility that the issue is with a particular model)? Has there been a Firefox update recently (is your copy set to install them silently in the background)? What about Windows updates? If you have any Firefox extensions installed, have you tried turning them off to see whether there could be some interaction between them and revised 3DW code?
You could try opening the Firefox Web Console (ctrl-shift-K) when you access the 3DW. It will list a lot of technical stuff, much of which won’t reveal anything useful. It will likely list warnings, most of which are harmless but please report them here. But if it lists errors, they may reveal incompatibilities between the 3DW content and your specific Firefox version/configuration. The 3DW staff would surely like to know about these, as it is very difficult to test a web site against every possible browser and configuration!
These results together indicate a problem with your installation of IE, so let’s explore that. If you can get IE to work externally, it should also work from within SU. When you say “will not even open”, what exactly happens? IE freezes/spins? Opens a blank page? Posts an error message? Shows the 3DW page but it is unresponsive? Can’t sign in?
Have you updated to the latest version of IE 11? Have you cleared the IE cache and cookies? What security settings do you have? Does IE have a saved userid and password it may be attempting to use?
Please let us know what you try and what you find, as it may help others with similar setups to yours.