Only ports 80 and 443 are supported for either inbound or outbound traffic. On most instances, the javascript and css content are downloaded from a Content Distribution Network in an attempt to improve performance for users with a cold cache. This strategy is usually a win, but occasionally there have been cases where the CDN caused worse performance or a problem with its content was inappropriately cached, resulting in pages failing to render correctly for a subset of users.
If only some users are affected, then it probably makes sense to record the session as a HAR file and contact support so that they can investigate. Firewalls don't seem likely to be involved here unless you're blocking access to the CDN, which doesn't seem likely and would impact all of your users rather than specific ones.
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