Firefox itself isn't. But configuring the UI and getting some functionality in is. (Some) extensions are huge bundles of functionality, affecting things all over the place. It's not a fault of firefox, but firefox caused it by not having very basic functionality built in (namely tab ordering, cycling, ...).

Then there are frustrating things like how do I move the reload/stop button to the left? Why do have quite powerful UI customization when it doesn't allow such a simple operation?