It’s not just “older browsers”, it doesn’t work at present with the latest Firefox either. It’s still the same issue that was reported back in January:
where your CDN is doing a 301 redirect for the SVG icons to a URL that is not permitted by the Content-Security-Policy. You may need to reach out to Discourse again, or see if you can add https://community.quickfile.co.uk/svg-sprite/ to the list of allowed script-src in the Discourse settings.