Firefox has a powerful feature that lets you search specialized sites from the location bar, called Search Keywords. Normally, when you type something in the location bar, and it isn't recognized as an address, Firefox will open up a Google search page for the string you typed. The idea is you can type wiki optical flow and you'll be directed to wikipedia's internal search results for optical flow.
In theory, all you need to do to set up custom keywords for the sites you visit most often is navigate to the main page, right-click on the search box and choose Add a Keyword for this Search from the menu.
In practice, this GUI method doesn't work on a lot of sites, since Firefox seems to have trouble with relative URLs in search forms. So doing this on wikipedia will send you to http://wiki/Special:Search?search=optical+flow rather than the full address!
There's some handy pre-packaged collections you can download, such as this one from Lifehacker, or this massive compendium, but if you're comfortable building up URLs, I'll show how you can create your own.
We start by doing a search on the site we want to add, and creating a bookmark of that page. In this case, we go to http://en.wikipedia.org and enter optical flow in the search box on the left, and press the Search button (not Go, since that's the equrivalent of I Feel Lucky on Google). That takes us to this page:
Pressing Control+D, or choosing Bookmarks->Bookmark This Page from the main menu adds this to our bookmarks.
Now, bring up the Bookmark Organizer by picking Bookmarks->Organize Bookmarks... from the menu. Find the bookmark you just created in the list, select it, and press Control+I or click the Properties icon in the top bar.
We should now see a dialog box like this:
In the location text box, scroll until you see the words you entered as a search, separated by a plus sign:
This is the part we'll want to replace with the terms after the keyword in the location bar, so delete just those words and any plus's separating them, making sure not to delete past the words. We'll delete up to the end of the URL, or to the next & sign, whichever comes first.
Once the specific terms have been removed, we can put in the placeholder for the terms we type after the keyword, %s
Finally, we chose a keyword to type for this search, in our case wiki, and click OK.
Now, we can test out the keyword by typing wiki gaussian blur in the location bar, and we should see a new page of search results.
This is a real time-saver if you set it up for reference sites you commonly search on, sometimes you can't beat the command-line style for speed of access!