FF 4.0 Fix Rated 5 out of 5 stars
1-Click Weather is great. It's easily customizable, the icons are smaller than Firefox's standard icons and take up little space, and it's updated constantly by its links to Weather.com. I've used it with FF for as long as I can remember. The low average rating is completely unjustified.
Several reviews below link to updated versions for FF 4.0, or to directions for fixing it yourself. See the review by Silverhawk, Blechtrottel, and Bilko, for example. (Thanks to you all!) I had no problem re-installing it after upgrading to FF 4.0.