Here's how to do it from the google toolbar.
How do I delete my search history?

Your Google search history can be deleted from the Google Toolbar, from the Google search box, and from your browser. Instructions for each of these instances appear below:

CLEARING YOUR GOOGLE TOOLBAR HISTORY

To clear your search history from the drop-down search box in the Google Toolbar for Firefox:

1. Click on the drop-down arrow to the right of the Google Toolbar search box.
2. Select the "Clear Search History" option. This will clear all the previous searches from the drop-down box.

Please note that this only clears the search history from the Google Toolbar, which is entirely separate from the search history stored by your browser.

CLEARING YOUR GOOGLE SEARCH BOX HISTORY

The search history that displays in the search box on the Google homepage is stored by your browser, not by Google. To disable this feature in Mozilla Firefox 1.5:

1. Go to the "Tools" menu.
2. Select "Options" > "Privacy" > "Saved Form Information."
3. Click "Clear" and un-check the box.
4. Click "OK" to exit.

To disable this feature in Mozilla Firefox 2:

1. Go to the "Tools" menu.
2. Select "Options" > "Privacy."
3. Click "Clear Now"... and make sure the box next to "Saved Form and Search History" is checked.
4. Click "Clear Private Data Now" to exit.

CLEARING YOUR BROWSER HISTORY

To clear your history from Mozilla Firefox 1.5:

1. Go to the "Tools" menu.
2. Select "Options" > "Privacy" > "History."
3. Click on "Clear." To completely disable the history function, set "Remember visited pages for the last X days" to zero ("0").
4. Click "OK" to exit.

To clear your history from Mozilla Firefox 2:

1. Go to the "Tools" menu.
2. Select "Options" > "Privacy."
3. Click "Clear Now"... and make sure the box next to "Browsing History" is checked.
4. Click "Clear Private Data Now" to exit.

For further assistance, you can find answers to some of Google's most frequently asked questions at http://www.google.com/help/index.html