I don’t like to install a lot of toolbars in Firefox (or Opera) because they tend to decrease the screen size that is available for the website. Sure, there is a way to display and hide the toolbars whenever I need them but this is not that practical and I tend to forget about them pretty fast.
Digital Inspiration, one of my favorite blogs, discovered a way to use Stumbleupon without installing the toolbar. This means that everyone, no matter which browser he is using, may use Stumbleupon to discover new and interesting websites and use most of the functions like commenting on a website, voting for it and so on.
It all comes down to the url stumbleupon.com/demo/#url=http://www.ghacks.net/ which will open a small toolbar on top of the website with the common features known from Stumbleupon. Just change the url from my url to one that you prefer.

Clicking on the Stumble button opens a random website from all stored websites at Stumbleupon. I like this gives a vote for the current website if you have an account at Stumbleupon and Send to a friend lets you send the website to a friend. The four buttons afterwards are more interesting. Stumble Websites, Photos, Videos or People.
The bar will remain if you click on a button letting you stumble websites for as long as you like. Wonderful way to discover new websites.
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another cool way to stumble is to install the toolbar, assign shortcuts, then hide the toolbar ;-)
works perfectly for me