Why is RM Addon failing in some artifact formats in firefox?

org.eclipse.emf.common.util.WrappedException: java.net.ConnectException: Connection timed out: connect
    at com.ibm.rdm.emf.resource.common.CommonResourceSetImpl.delegatedGetResource(CommonResourceSetImpl.java:126)
    at org.eclipse.emf.ecore.resource.impl.ResourceSetImpl.getResource(ResourceSetImpl.java:379)
    at com.ibm.rdm.ui.gef.editmodel.EditModel.getResource(EditModel.java:151)
    at com.ibm.sid.ui.editmodel.SketchingEditModel.getResource(SketchingEditModel.java:95)
    at com.ibm.sid.ui.editmodel.SketchingEditModel.getResource(SketchingEditModel.java:1)
    at com.ibm.rdm.ui.gef.contexts.RootContext.getResource(RootContext.java:125)
    at com.ibm.rdm.ui.gef.contexts.RootContext.init(RootContext.java:86)
    at com.ibm.sid.ui.screenflow.contexts.RootScreenFlowContext.init(RootScreenFlowContext.java:39)
    at com.ibm.sid.ui.screenflow.ex.contexts.RPCRootScreenFlowContext.init(RPCRootScreenFlowContext.java:119)
    at com.ibm.rdm.ui.gef.contexts.RootContextEditor.setEditModel(RootContextEditor.java:244)
    at com.ibm.sid.ui.screenflow.ex.parts.RPCScreenFlowEditor.setEditModel(RPCScreenFlowEditor.java:40)
    at com.ibm.rdm.ui.gef.contexts.RootContextEditor.init(RootContextEditor.java:219)
    at org.eclipse.ui.internal.EditorManager.createSite(EditorManager.java:798)
    at org.eclipse.ui.internal.EditorReference.createPartHelper(EditorReference.java:647)
    at org.eclipse.ui.internal.EditorReference.createPart(EditorReference.java:465)
    at org.eclipse.ui.internal.WorkbenchPartReference.getPart(WorkbenchPartReference.java:595)
    at org.eclipse.ui.internal.PartPane.setVisible(PartPane.java:313)
    at org.eclipse.ui.internal.presentations.PresentablePart.setVisible(PresentablePart.java:180)
    at org.eclipse.ui.internal.presentations.util.PresentablePartFolder.select(PresentablePartFolder.java:270)
    at org.eclipse.ui.internal.presentations.util.LeftToRightTabOrder.select(LeftToRightTabOrder.java:65)
    at org.eclipse.ui.internal.presentations.util.TabbedStackPresentation.selectPart(TabbedStackPresentation.java:473)
    at org.eclipse.ui.internal.PartStack.refreshPresentationSelection(PartStack.java:1254)
    at org.eclipse.ui.internal.PartStack.setSelection(PartStack.java:1207)
    at org.eclipse.ui.internal.PartStack.showPart(PartStack.java:1606)
    at org.eclipse.ui.internal.PartStack.add(PartStack.java:497)
    at org.eclipse.ui.internal.EditorStack.add(EditorStack.java:103)
    at org.eclipse.ui.internal.PartStack.add(PartStack.java:483)
    at org.eclipse.ui.internal.EditorStack.add(EditorStack.java:112)
    at org.eclipse.ui.internal.EditorSashContainer.addEditor(EditorSashContainer.java:63)
    at org.eclipse.ui.internal.EditorAreaHelper.addToLayout(EditorAreaHelper.java:225)
    at org.eclipse.ui.internal.EditorAreaHelper.addEditor(EditorAreaHelper.java:213)
    at org.eclipse.ui.internal.EditorManager.createEditorTab(EditorManager.java:778)
    at org.eclipse.ui.internal.EditorManager.openEditorFromDescriptor(EditorManager.java:677)
    at org.eclipse.ui.internal.EditorManager.openEditor(EditorManager.java:638)
    at org.eclipse.ui.internal.WorkbenchPage.busyOpenEditorBatched(WorkbenchPage.java:2860)
    at org.eclipse.ui.internal.WorkbenchPage.busyOpenEditor(WorkbenchPage.java:2768)
    at org.eclipse.ui.internal.WorkbenchPage.access$11(WorkbenchPage.java:2760)
    at org.eclipse.ui.internal.WorkbenchPage$10.run(WorkbenchPage.java:2711)
    at org.eclipse.swt.custom.BusyIndicator.showWhile(BusyIndicator.java:70)
    at org.eclipse.ui.internal.WorkbenchPage.openEditor(WorkbenchPage.java:2707)
    at org.eclipse.ui.internal.WorkbenchPage.openEditor(WorkbenchPage.java:2691)
    at org.eclipse.ui.internal.WorkbenchPage.openEditor(WorkbenchPage.java:2674)
    at com.ibm.rdm.rrcx.ui.application.workbench.editors.EditorOpener$1.run(EditorOpener.java:102)
    at org.eclipse.swt.widgets.RunnableLock.run(RunnableLock.java:35)
    at org.eclipse.swt.widgets.Synchronizer.runAsyncMessages(Synchronizer.java:134)
    at org.eclipse.swt.widgets.Display.runAsyncMessages(Display.java:4041)
    at org.eclipse.swt.widgets.Display.readAndDispatch(Display.java:3660)
    at org.eclipse.ui.internal.Workbench.runEventLoop(Workbench.java:2640)
    at org.eclipse.ui.internal.Workbench.runUI(Workbench.java:2604)
    at org.eclipse.ui.internal.Workbench.access$4(Workbench.java:2438)
    at org.eclipse.ui.internal.Workbench$7.run(Workbench.java:671)
    at org.eclipse.core.databinding.observable.Realm.runWithDefault(Realm.java:332)
    at org.eclipse.ui.internal.Workbench.createAndRunWorkbench(Workbench.java:664)
    at org.eclipse.ui.PlatformUI.createAndRunWorkbench(PlatformUI.java:149)
    at com.ibm.rdm.rrcx.ui.application.PluginWindowFactoryService$1.run(PluginWindowFactoryService.java:138)
    at java.lang.Thread.run(Thread.java:767)
    at org.eclipse.swt.widgets.Synchronizer.syncExec(Synchronizer.java:179)
    at org.eclipse.swt.widgets.Display.syncExec(Display.java:4584)
    at com.ibm.rdm.rrcx.ui.application.PluginWindowFactoryService.openResource(PluginWindowFactoryService.java:211)
Caused by: java.net.ConnectException: Connection timed out: connect
    at java.net.PlainSocketImpl.socketConnect(Native Method)
    at java.net.PlainSocketImpl.doConnect(PlainSocketImpl.java:412)
    at java.net.PlainSocketImpl.connectToAddress(PlainSocketImpl.java:271)
    at java.net.PlainSocketImpl.connect(PlainSocketImpl.java:258)
    at java.net.SocksSocketImpl.connect(SocksSocketImpl.java:376)
    at java.net.Socket.connect(Socket.java:546)
    at com.ibm.jsse2.SSLSocketImpl.connect(SSLSocketImpl.java:567)
    at org.apache.http.conn.ssl.SSLSocketFactory.connectSocket(SSLSocketFactory.java:524)
    at org.apache.http.conn.ssl.SSLSocketFactory.connectSocket(SSLSocketFactory.java:403)
    at org.apache.http.impl.conn.DefaultClientConnectionOperator.openConnection(DefaultClientConnectionOperator.java:177)
    at org.apache.http.impl.conn.AbstractPoolEntry.open(AbstractPoolEntry.java:144)
    at org.apache.http.impl.conn.AbstractPooledConnAdapter.open(AbstractPooledConnAdapter.java:131)
    at org.apache.http.impl.client.DefaultRequestDirector.tryConnect(DefaultRequestDirector.java:611)
    at org.apache.http.impl.client.DefaultRequestDirector.execute(DefaultRequestDirector.java:446)
    at com.ibm.team.jfs.app.http.client.internal.ExtensibleRequestDirector.execute(ExtensibleRequestDirector.java:168)
    at org.apache.http.impl.client.AbstractHttpClient.doExecute(AbstractHttpClient.java:863)
    at org.apache.http.impl.client.CloseableHttpClient.execute(CloseableHttpClient.java:82)
    at org.apache.http.impl.client.CloseableHttpClient.execute(CloseableHttpClient.java:106)
    at com.ibm.rdm.client.api.RRCRestClient.executeHttpMethod(RRCRestClient.java:453)
    at com.ibm.rdm.client.api.RRCRestClient.performGet(RRCRestClient.java:143)
    at com.ibm.rdm.client.api.BaseJenaServiceClient$RRCRestClientDelegate$1.performGet(BaseJenaServiceClient.java:42)
    at com.ibm.rdm.client.api.BaseJenaServiceClient.getModel(BaseJenaServiceClient.java:273)
    at com.ibm.rdm.client.api.BaseJenaServiceClient.getModel(BaseJenaServiceClient.java:242)
    at com.ibm.rdm.emf.resource.common.CommonURIHandlerImpl.createInputStream(CommonURIHandlerImpl.java:213)
    at com.ibm.rdm.emf.resource.common.CommonURIHandlerImpl.contentDescription(CommonURIHandlerImpl.java:144)
    at org.eclipse.emf.ecore.resource.impl.ExtensibleURIConverterImpl.contentDescription(ExtensibleURIConverterImpl.java:313)
    at com.ibm.rdm.emf.resource.common.CommonResourceSetImpl.delegatedGetResource(CommonResourceSetImpl.java:121)

It works on Internet Explorer, so it is not a generalized problem with the server.