This extension pulls quotes from Bloomberg, Google Finance and Yahoo! Finance. All of these services have their strengths and weaknesses with market coverage, charting tools and personal portfolios, so why choose between them? The quotes you've chosen update automatically, and clicking on one will open its price history chart in the same window.
To add your own ticker symbols, just visit the options page (accessible via both the right-click menu and the popup window). Instructions are there, although it mostly just involves you typing a ticker symbol and selecting the quote service you wish to use. You can also edit which metrics you want to see in the columns displayed, as well as the update frequency.
As of version 1.3, I've added the ability to put in the price paid and shares owned for your portfolio so you can track your performance. Also, you can now add alerts if the price falls above or below a specified level. And for those who want to separate things by portfolio or market, you can create different portfolios.
The default portfolio is there for demonstration: you can track domestic and foreign stocks, 3-month LIBOR, exchange rates, swap rates, indexes tracking everything from the broad market to the dollar to volatility, and futures. Heck, I've even thrown in the 5Y senior EUR CDS on Greece. You can clear everything in the options screen with the Clear button and start your own.
Note that for some securities, certain price data and charts aren't available.