Fastest Weather Forecast is the easiest way to check the weather while you browse internet.
Here is a little more about the add-on from the developers:
Weather Forecast is a handy and reliable extension designed to notify you about the current weather condition in your area (or any other locations) right in your browser
1. Multiple locations support
2. See the complete Weather.com mobile site in a fancy Panel
3. Shows the current weather temperature in the toolbar icon
Want to learn more? You can pick up this add-on for Firefox at the Firefox Add-ons website.