[Plugin-DropDown Menu] Blocked loading mixed active content -SSL support

I have checked and encountered following error in Firefox while loading the homepage:
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Blocked loading mixed active content “http://[mydomain.com]/wp-content/plugins/dropdown-menu-widget/themes/web20.css”
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“Dropdown Menu Widget” plugin which is referencing the css file over a non-secure connection.
How can I make dropdown menu work on ssl domain, Any solution would be helpful?