How to Use Web Development Toolbar in Mozilla Firefox Open firefox browser on your computer. Right click on the empty space and select menu bar. This displays menu bar. Now go to tools and click add-ons. Click on the get add-ons tab. Search for web developer add-on in the search box. Click the install button. Restart the firefox to complete the installation. Navigate to the site whose source code and style sheet you would like to view. Click “View Source” on the tool bar and click “View Source” on the menu that drops down. A new window pops up with the HTML source code ofthe current page. Next to view the style sheet for the page,Click “CSS” on the tool bar. Click “View CSS” on the menu that drops down. Notice that a new tab opens in Firefox with the style sheet code. Next to see how you website looks in different devices, click “resizer” on the tool bar and click “View responsive laayouts”. In Similar way, Experiment with other options in the tool bar. This is How you can use the Web Developement tool bar in mozilla firefox.