When I tried out FF 53, I discovered that some of my add-ons got disabled. The problem was with "unsigned" add-ons. Usually these were legacy add-ons that continued to be maintained after Mozilla banned pre-WebEx add-ons, and thus could no longer get Mozilla to sign them. The biggest example was the legacy version of uBlock Origin, which I use because it offers more privacy protections than the WebEx version does on FF 52 and 53. (A couple were add-ons that were originally signed, but from which I had removed the signature in order to implement some hack.)
In FF 52 ESR, unsigned add-ons aren't much of a problem. You simply set the about:config preference xpinstall.signatures.required to false and you're good to go. The unsigned add-ons will produce warnings in the about:addons page, but they work. You can even hide the warnings with an add-on like Classic Theme Restorer.
But FF 53 is a "stable" release, and xpinstall.signatures.required doesn't work in stable FF releases. Luckily, there is a workaround, but it's a bit more complex than just setting a preference.
The workaround at the link can be combined with @VistaLover's fix for re-enabling SSUAOs in Firefox 52:
First, follow the instructions there; then add this JavaScript to the config.js file you just created:
try {
Components.utils.import("resource://gre/modules/addons/XPIProvider.jsm", {})
.eval("SIGNED_TYPES.clear()");
}
catch(ex) {}
(I put it after @VistaLover's code, but it will probably work before it too.)
One last thing. Since FF 53 doesn't expect to have add-on signing disabled, it doesn't have the yellow "warning" text that FF 52 ESR does for an "unverified" add-on. Instead, you'll see a scarier, red "This has been disabled" message. However, it has not been disabled; you'll still see a "disable" button which wouldn't be there if the add-on were already disabled. Luckily, hiding warnings (with, e.g., the Classic Theme Restorer add-on) will hide these scarier messages just as it hides the yellow warnings in FF 52 ESR.