Directly drag text into the search bar
The search bar is located at the upper right corner of Firefox and can be focused using Ctrl+K. By default, the search engine used is Google. Instead of hitting Ctrl+K then entering a piece of text there and pressing Enter, you can select the text on the currently loaded web page and drag it into the text box. Firefox will automatically search on the default engine for the phrase you dragged there. The only drawback to this is that the results page is loaded in the same, current tab.
Add tags to bookmarks to fasten searches in the awesome bar
One of the new features in Firefox 3.0 was the improved location bar, called Awesome Bar in Firefox, which allows you to search for a website in your bookmarks or browsing history just by entering word(s), in any order, which are included in the web address or title of the web page. The search also occurs for bookmark tags, so one way to fasten your searches in the awesome bar is to add tags to your bookmarks.
To do that, just go to the Bookmarks menu, right click on the bookmark of choice, go to Properties and fill in the Keyword field with tags separated by commas. Another way is to go to Bookmarks -> Organize Bookmarks and add tags there.