Opera do supports plugins and widgets but ofcourse not as popular and versatile as the plugins of firefox...
I think popularity also depends on the marketing strategies.. Firefox advantage is it comes as default with Linux like Ubuntu!!!
Now Google is introducing new features into it's social...
Make Sitemaps, Generate MetaTags by urself or by third party tools, Make RSS Feed to tell users about ur latest changes. Submit ur site to popular search engines like Google,Yahoo , MSN .