Over the span of time from I found Firefox (when it was still called Firebird) till now, I have found several wonderful extensions. Here are a few I found recently.
I particularly like the Inline MP3 Player which “Inserts an inline play button(flash plugin required) after every link on page that ends with a .mp3 extension.”
This extension is a wonderful way to see just how a website ticks. You can see just how it works with a demo right on the website. Here’s what they say about this extension:
As you glide the mouse over the page, you will see a red rectangle framing each element under the cursor. You will also see a little yellow caption showing the HTML element type and its class or id if they exist.
There are still more stuff they are planning for this extension but even until then, I highly recommend it for those who are curious how a particular webpage was made.