Replace the <RibbonDiffXml> with the below code.It will add the button on Account main form with javascript code
<RibbonDiffXml>
<CustomActions>
<CustomAction Id="CA_MyFirstButton" Location="Mscrm.Form.account.MainTab.Save.Controls._children" Sequence="31">
<CommandUIDefinition>
<Button Id="B_MyFirstButton" Command="MSCRM.Form.account.MainTab.Save.Controls._children" LabelText="My First Button" ToolTipTitle="My First Button Tool Tip Title" ToolTipDescription="My First Button Tool Tip Description"
TemplateAlias="o1" Image16by16="/_imgs/ribbon/saveandclose16.png" Image32by32="/_imgs/ribbon/saveandclose32.png" /></CommandUIDefinition>
</CustomAction>
</CustomActions>
<Templates>
<RibbonTemplates
Id="Mscrm.Templates"/>
</Templates>
<CommandDefinitions>
<CommandDefinition Id="MSCRM.Form.account.MainTab.Save.Controls._children">
<EnableRules />
<DisplayRules />
<Actions>
<JavaScriptFunction FunctionName="hello" Library="$webresource:new_test">
<CrmParameter Value="CommandProperties"></CrmParameter>
</JavaScriptFunction>
</Actions>
</CommandDefinition>
</CommandDefinitions>
<RuleDefinitions>
<TabDisplayRules/>
<DisplayRules/>
<EnableRules/>
</RuleDefinitions>
<LocLabels/>
</RibbonDiffXml>
create a web resource test and add the below function
function hello(){
alert("My First Button with Custom button");
}
whenever you click on the custom button it shows an alert
