Remember that Firefox was originally intended as a minimalist platform, with the expectation that users would use its extension mechanism to build it up according to their needs. As a result, Firefox's extension mechanism is quite liberal. The upside is that its extensions are capable of much more than, for example, addons in Chrome. The downside is that extensions are responsible for much more, and can touch basically any part of the program (which is why powerful addons are so prone to breaking with new versions of Firefox: they're effectively targeting an unstable API).

Chrome addons are less prone to breakage and mischief, because Chrome addons target a stable API with a limited set of functionality. Firefox introduced just such an API last year, called the Jetpack API (it may have been renamed since then). You can tell if an extension is using the Jetpack API if it doesn't require a browser restart to install.