For the longest time I had used the Weather Bug extension as I was a subscriber to their Windows desktop widget. However, once Firefox 4 was released the extension stopped working. For a long while their website said they were working on getting it operational for Firefox 4 and newer. Finally though, they announced they were dropping the extension. So I started looking for another Weather extension. I wanted something that was low profile and customizable.
After looking around AMO, I came across Forecastfox Weather, powered by Accuweather.com. There are a lot of things about this add-on that impress me. The biggest is how customizable it is. You can choose which icon set to use, days, nights or day & nights, how many days (0-6) to include in the forecast and lots more options. Another nice feature is the radar, simply mouse-over the radar icon and the radar pop-ups (you can choose small, medium, large or choose a custom size), no having to wait for it to open and load in a new tab/window. You can also place Forecastfox on any toolbar or even on the
status add-on bar. You can also setup to display forecasts for multiple locations including international.
While the add-on is a little hefty at 450 KB (there is a lot to this), it does not cause Firefox to consume much more memory. The only complaint I have is the options should be available via the Tools Menu (Edit for Linux/Mac) instead you have to go into the add-ons manager click options and then the control panel opens in a new tab. This add-on works with Firefox 3.6 up to and including the current nightly (10.0a1). Also, they have a (crippled) version of this add-on for Google Chrome.