I intend to modify the display of existing websites.
For example, when Google returns search results, can I customize the web page displaying (such as adding some bubble visualizations onto that page's blank places) ? Could it be done using browser plug-in or extension?
Based on my knowledge, a browser extension, such as firefox or chrome extension, is usually for adding tiny icons to the address bar.
Could it be done using browser plug-in or extension?
Yes. But you don't want to write a browser plug-in. They're fairly complicated to write and users are reticent to install them (with good reason) Further, you'd have to write two, as Mozilla and Google can't agree on a format (Chrome is removing the venerable NPAPI, but Mozilla won't implement Google's PPAPI, claiming that it's a moving target with inadequate documentation).
Based on my knowledge, a browser extension, such as firefox or chrome extension, is usually for adding tiny icons to the address bar.
No. Firefox add-ins and Chrome extensions can do a lot more than that, including modifying the page once it's rendered.
So you'll probably want to write a Firefox add-in and a Chrome extension. (Sadly, yes, that's two codebases to maintain, although they use similar technologies — HTML, CSS, JavaScript — so you can share a bit between them.)