For security purposes, Gecko places some restrictions on SVG content when it's being used as an image:

• JavaScript is disabled.
• External resources (e.g. images, stylesheets) cannot be loaded, though they can be used if inlined through data: Ls.
• :visited-link styles aren't rendered.
• Platform-native widget styling (based on OS theme) is disabled.

Note that the above restrictions are specific to image contexts; they don't apply when SVG content is viewed directly, or when it's embedded as a document via the <iframe>, <object>, or <embed> elements.