Fieldsets with the following config under FF have incorrect layouts

Chrome and IE render/layout the Fieldsets correctly

Note: if you add "width : 200" to the Fieldsets it corrects the issue in FF to a fashion. i.e. the layout can be seen to reposition its self.

See attached images

Test Case
Code:
Ext.onReady(function(){

	myForm = Ext.widget('form', {
		title: 'Test Title',
		border: 0,
        width: 481,
        bodyPadding: 0,
        fieldDefaults: {
            labelAlign: 'left',
            labelWidth: 90,
            msgTarget: 'qtip'
        },
		items: [
			{
				xtype: 'fieldset',
                title: 'Fieldset 1',
                defaultType: 'textfield',
				collapsible: true,
				collapsed: true,
                layout: 'anchor',
                defaults: {
                    anchor: '100%',
					labelWidth: 100
        		},
				items: [
					{
						xtype: 'container',
						layout: 'hbox',
						defaults: {
							margins: '0 5 0 5'
						},
						items: [
							{
								xtype: 'fieldset',
								title: 'Sub Fieldset 1',
								layout: 'anchor',
								flex : 1,
								defaults: {
									anchor: '100%'
								},
								items: [
									{
										xtype: 'numberfield',
										fieldLabel: 'Testing'
									},
									{
										xtype: 'numberfield',
										fieldLabel: 'Testing'
									}

								]
							},
							{
								xtype: 'fieldset',
								title: 'Sub Fieldset 2',
								layout: 'anchor',
								flex : 1,
								defaults: {
									anchor: '100%'
								},
								items: [
									{
										xtype: 'numberfield',
										fieldLabel: 'Testing'
										
									},
									{
										xtype: 'numberfield',
										fieldLabel: 'Testing'
									}
								]
							}
						
						]
					}
				]
				
			}
		]
	});



	var myPanel = Ext.create('Ext.panel.Panel', {
		region: 'center',
		id: 'myPanelId',
		border: 0,
		items: [
			myForm
		]
		
	});

	Ext.create('Ext.Viewport', {
        id: 'testing',
		layout: {
            type: 'fit'
        },
        items: [
			myPanel
		]
	});

});
FF 3.6.16
FF.jpg

Chrome
Chrome.jpg