Adblock Plus is a terrific extension for Firefox, along with the EasyList rule set. One minor problem I've run into recently, however, is that EasyList blocks the automatic package-tracking links that appear in the sidebar in GMail (when viewing emails that contain a tracking number). I found the offending rule in the list and disabled it, allowing me to get my links back. Here's how to do it:

1. Open the Adblock Plus Preferences dialog (Tools » Adblock Plus Preferences)
2. Press Ctrl + F to open the find bar
3. Search for the following text (only one rule should match it): &view=ad
4. Disable said rule

The entire rule looks like this, in case you're curious: ||mail.google.com/mail/*&view=ad