After you have your Workbench instance setup, download Workbench Tools
for Google Chrome from http://forceworkbench.googlecode.com/files/workbenchtoolsforchrome-1.0.0.crx.
Now, when you are logged into Salesforce, the blue Workbench logo will
be displayed in your address bar. Simply click on this button and
you'll be automatically logged into Workbench.
The first time you use the extension, you will be prompted to provide
your Workbench base URL. For example, if the login page for your
Workbench instance is at http://localhost/workbench/login.php, just
enter http://localhost/workbench. If you need to access the Options
menu in the future, right-click on the toolbar button. Note, this
browser extension is not compatible with the public demo site at
workbench.developerforce.com due to terms of service agreement and
login CSRF requirements.
Workbench Tools for Firefox is also available at:
https://addons.mozilla.org/en-US/firefox/addon/235866/?src=external-googlecode