This script sorts your watchlist by namespace, and adds spaces within each entry. This makes the watchlist much easier to read.
To install the script, copy/paste it to your your common.js page. This will install the script across all your skins.
Note: After saving, you have to bypass Firefox's cache to see the changes. To do this, hold down the ⇧ Shift key while clicking Reload (or press Ctrl+⇧ Shift+R). Or, you may have to purge the Wikipedia server cache.