fun fact: it's easy to make a userscript like i do!
steps:
1. go to a desired website (or use a website feature) that doesnt work in tenfourfox/iwppc
2. if a website/feature doesnt work, go to browser console and check for example
ReferenceError: AbortController is not defined
3. go to npmjs.com and find a polyfill for the javascript function that is missing
4. once you found it, go to unpkg.com/<package name>, such as
https://unpkg.com/@ungap/custom-elements
5. create a userscript then put the contents of that polyfill inside it
6. save the userscript (cmd+s), then reload the page and see if the affected website/function works now
7. if it doesn't work, go back to npmjs and search other polyfills for the same javascript function
8. good luck!