A couple of things have helped me solve issues when trying to load sites in Firefox that will work in Chrome/Vivaldi, etc.:
1.) Refresh Firefox: Click the menu button with 3 lines -> "Help" -> "More troubleshooting information" -> "Refresh Firefox..."
2.) Check your Enhanced Tracking Protection settings from the Privacy & Security tab in the Settings menu. If it's set higher than Standard, it could be causing sites like reddit, sites that use Cloudflare for protection, etc., to load incorrectly or fail to load entirely.