This page lists the three known reasons that your browser might not execute JavaScript which the server sent to it. They are listed in order of estimated "limited relative" frequency.
If you are wondering why Wikia pages which require JavaScript are not working but other such (non-Wikia) pages are, see Help:Customization.
Disabled Settings
To combat malicious JavaScript, some browsers offer a setting which will always cause JavaScript (or only some of it) to be ignored. If you want JavaScript-dependent pages (including your own) to work, you obviously must have it, or at least the features which you are interested in, enabled. The steps to enable it vary by browser; if your browser is not listed here, please refer to its documentation and edit the information (but no copyrighted text) into this page.
Mozilla Firefox
Open the Tools > Options or (on Mac and Linux) Edit > Preferences menu, then the Content tab. Check the Enable JavaScript checkbox. Click the Advanced... button and check/uncheck the boxes according to your personal preferences. See also the Firefox Help article.
Incapable Browsers
Antique browsers written before the advent of JavaScript will ignore it, or even display it as text. Additionally, some browsers only support an obsolete version of the ECMAScript standard.
Antique browsers (those which support no JavaScript) include Internet Explorer v<3.0, Netscape Navigator v<2.0, Opera v<6.0, and Safari v<3.0[1].
Meddling Proxies
Some proxy servers, primarily corporate firewalls, strip all JavaScript from incoming pages as a security measure. The only solutions available for this problem are to not use the proxy and to get its settings changed.