Firefox will send in some cases no request header "If-Modified-Since", in that case Jira will send back status 304 and body.

In atlassian-plugins-webresource class com.atlassian.plugin.webresource.AbstractBatchDownloadableResource method isResourceModified.

If one of the resources is modified and one is not, com.atlassian.plugin.servlet.util.LastModifiedHandler.checkRequest set for one resource "response.setStatus(HttpServletResponse.SC_NOT_MODIFIED)" and the method isResourceModified will return true.

This makes com.atlassian.plugin.servlet.PluginResourceDownload will return the body in the method serveFile. Which must not happen as per this guide (section 10.3.5)