Tried most weather addons - Weather Extension has the edge.
The layout is sleek, professional looking, offering an intuitive forecast at a glance which works well with dark and light Firefox themes.
Features like future-forecasting, f°/c°, dark/light themes and links to full page maps are included for free, and there's no intrusive nagging to upgrade. The extended customisation options and alerts are well worth the price of the premium version.
In terms of additional features, I'd love to see a one click option to view the forecast and radar in the same page. But I can spare time for two clicks :D