The exact steps to enable JavaScript vary from one browser to the next (and from one version of the same browser to the next) but basically the procedure is as follows:
1. How to Turn JavaScript on for Firefox
a. From the Tools menu select Options.
b. When the Options dialog box opens, select the Content tab (shown in the next screenshot).

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c. Make sure that there is a tick in front of the Enable JavaScript option. If you want, you can configure some advanced options for JavaScript but even if you leave it as is and just click the OK button to save the changes, it is fine.
2. How to Turn JavaScript on for Internet Explorer
The procedure to turn JavaScript on in Internet Explorer is similar. Here is how to do it:
a. From the Tools menu select Internet Options.
b. When the Internet Options dialog box opens, select the Security tab and click the Custom Level button. This will open the Security Settings dialog box, shown in the next screenshot:

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c. Scroll down the list till you find the Scripting section and make sure that Active Scripting is enabled. Click the OK button to save the changes and that is it.