I'm not sure about userChrome.js. I am still planning to test that and will let you know. I'd been wanting to try out the add-on icon logic in the autoconfig file instead of using userChrome.js, but that didn't work due to the sandboxing.
I will probably keep FF ESR as a secondary browser (I'd been using the pre-quantum ESR for that but now I can at least update it to the latest ESR), and Waterfox as my primary. I like Waterfox and haven't had issues with it; my main concern with it is that it doesn't get the security updates as quick. But that's a risk I'm willing to take as long as FF keeps being annoying like this.
Re: Mozilla/Firefox did it again
Per this page:
https://malwaretips.com/threads/firefox-quantum-v62-0-released.86481/
There is currently a flag (general.config.sandbox_enabled) to disable the sandboxing, but it's temporary and won't be supported for long.
I will probably keep FF ESR as a secondary browser (I'd been using the pre-quantum ESR for that but now I can at least update it to the latest ESR), and Waterfox as my primary. I like Waterfox and haven't had issues with it; my main concern with it is that it doesn't get the security updates as quick. But that's a risk I'm willing to take as long as FF keeps being annoying like this.