Mozilla Addons pages seems less usable now.
On Addons pages.
Note:
Trying in different profiles
Testing to solve with (fail to find the good options)
Polly extension
Agent Switcher
Waterfox Classic is based on 6 years old Firefox.
So I can imagine a lot of new API is missing (as well as security patches).
These days even Firefox ESR based on Firefox 102 is sometimes “too old” for modern websites, and it’s not even one year old!
Anyway, using Waterfox classic can be dangerous, any malicious website can easily check what browser are you using and check the database for the known vulnerabilities and then serve you one - ideally some that doesn’t require user interaction.
Yes, i know…
But , “normally” be friendly with these “old” browsers and assure compatibility with them is the the good way.
And as i can know, these changes don’t give a big gain in functionality or what else i can notice when i use an Firefox Quantum, by example.
They Just break their use with “Old” browsers:
That’s the project ?
If i use an “Old” browser, that’s due, in some part, at the abandon of the XUL and its flexibility.
I never found yet with the famous Quantum.
About security:
I take care what i do with my surf and Waterfox Classic add these Security patches.
Anyway, thank you for the reply.
You can’t stop the progress 
New features are here for developers (requested by developers to make their life easier) so they love to use them whenever possible. I love to use them! (every September it’s like a Christmas for me because new Firefox ESR is released for everyone and I can start using all the new features introduced in the past year… new javascript features, new CSS features, new webextension API…)
Sure, you can export your page to be ES2015 compatible but that will generate huge bundle and can introduce new issues (some features can’t be easily polyfilled so they are buggy/incomplete/missing/etc…).
And for many developers it’s not worth the trouble. Some will only check that “it works in Chrome” and ignores Firefox completely 
. So your expectations for an old browser that doesn’t support modern standards should be very low.
