"TypeError: Intl.RelativeTimeFormat is not a constructor" on SeaMonkey

[dilworks]

Describe the bug
Not everybody uses mainline browsers (Chrome/Firefox), instead some of us prefer to use alternate browsers - in my case, SeaMonkey 2.53.10 (which is its latest version, released last week). Unfortunately, I can't open the store on it, as it dies with "TypeError: Intl.RelativeTimeFormat is not a constructor", because RelativeTimeFormat was introduced much later on the Firefox timeline (SeaMonkey is based off FF58 with many, MANY backports from later FF versions). I suspect this could affect Pale Moon users too.

To Reproduce
Steps to reproduce the behavior:

1. Load the store on SeaMonkey
2. See a unresponsive UI, and "TypeError: Intl.RelativeTimeFormat is not a constructor" logged on the developer console

Expected behavior
The store should load just fine

Desktop (please complete the following information):

• OS: Linux (Debian 10, amd64)
• SeaMonkey 2.53.1

Additional context
There is a polyfill available for older/legacy/alternate browsers, so it should be enough to include it to fix the site for us.
https://formatjs.io/docs/polyfills/intl-relativetimeformat
https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Intl/RelativeTimeFormat