Are you working as fast and smart as you could in your browser? One way to do that is with tabs. You'll learn about working with tabs in Firefox in this article. Most of these tips will also apply to Safari, Internet Explorer and Opera—though you may have to search the menus for the capability.
To open a page in a new tab means that you can have more than one browser window and web site open in a browser at a time. This is a time saver if you often use more than one site at a time. (My tabs are colored because I use a Firefox add-on called Colorful Tabs.) This image show several tabs open in my Firefox.

Using tabs means you don't have to use separate windows for each page you are viewing. It keeps everything in one place and let's you move from tab to tab without searching out a minimized or covered window. There are other advantages, too.
Lisa Williams suggested that having sites preloaded in tabs was a good technique when speaking in Give a Better Presentation!
Do not trust the local technology.
Do not assume that the Internet will work. Take screenshots or screen video of the sites you want to demonstrate. Less stable but quicker, load them as tabs in a tabbed browser and flip through the tabs as you talk (but if you accidentally click a link, you may not be able to reload if the internet connection is not good). Do not assume that you will be able to demo audio from your computer unless you specifically ask.
You can set the Firefox Preferences to open new pages in a new Tab. In Preferences, select the radio button for 'a new tab' under New pages should be opened in:

You can also open a page in a new tab, even if your Preferences aren't set that way, by right clicking (Ctrl-click on a Mac) and selecting 'Open link in new tab' from the contextual menu that pops up. Here's the contextual menu I see when I right-click a link.

If you have a folder of bookmarks, you can open them all at once in tabs. This is nice if you always open up the same set of pages when you start working. Make them all open quickly using bookmark folders and tabs.
An excellent article by Gina Trapani at Lifehacker tells How to Work with groups of tabs in Firefox. My favorite tip from this article is this one.
Bookmark a group of tabs into a folder. Open up the group of tabs - like all the sites you need to work on a specific project - and choose "Bookmark All Tabs" from the Bookmarks menu. (Or use the Shift+Ctrl+D keyboard shortcut (Shift+Cmd+D on the Mac.)) Then, set the folder name (like the name of your project.)
When you are ready to open the bookmarks in tabs, select the folder. Then find the link at the bottom of the set of bookmarks that says 'Open all in Tabs.' Select 'Open all in Tabs' and all the bookmarks in the folder will open immediately.

You can also save additional bookmarks to this folder once you have it set up. Simply choose Add Bookmark from the Bookmarks menu and make sure the new bookmark gets added to the folder you created.
If you are working with a set of tabs open and your computer or browser crashes or you close the browser for the night without bookmarking the group of tabs, you can set up Firefox to open with the same set of tabs the next time you use it. Set this up in Preferences. In the Main category, under Startup, use the pull down menu after When Firefox starts to select "Show my windows and tabs from last time."

You search with Google, right? You get a page of search results. You click one of them. The new page comes up and the search results are gone. Don't you hate that? You can set Google to open pages you click in your search results in a new window so that you can have multiple pages open in tabs and still have the page of search results there, too. Go to google.com. Beside the search box, click Preferences. Scroll down a bit to find 'Results Window.' Select the box for 'Open search results in a new browser window.' Now you'll get tabbed pages of search results that can be saved until you are finished with the search.
One caveat on the Google search, since I do this in Firefox and have the Preferences set to open new pages in a new tab, as I explained above, I get tabbed results for search pages. If you aren't running your browser this way, you might get a whole new browser window, which is not as useful as a new tab.
More resources to help you work smart with tabs:
MacTips: Tabbed Browser Tips for Safari browsers.
How to Handle Safari Tabs at KnowIT.
How to Work with Tabs in Firefox 3 an article of mine on eHow.
Firefox tutorial: Sorting your bookmarks at Pop ups of my Mind.
Browser Settings covers most major browsers—from History of Science Online.
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Virginia DeBolt
BlogHer Technology Contributing Editor
Web Teacher
First 50 Words