You can do this in the following way:

Mozilla Firefox (1.0)

• Select Tools from the top menu.
• Choose Options.
• Choose Web Features from the left navigation bar.
• Select the checkbox next to Enable JavaScript and click OK.

Mozilla Firefox (2.x)

• Open Firefox.
• On the Tools menu, click Options.
• Click Content in the Options list.
• Under the Content section, check the box next to Enable JavaScript.
• Click the Advanced button to open the Advanced JavaScript Options box.
• Check the appropriate boxes under Allow scripts to.
• Click OK.
• Click OK.