Rated 5 out of 5 stars
This solves my pet peeve... an e-mail that shows the tracking number, but does not create a link.
Although sometimes I have the problem of getting a link in an e-mail that sends me to a page with a tracking number as plain text - and this extension for Firefox solves it beautifully, more often I get a plain text tracking number in the body of the e-mail itself.
This extension works great in Thunderbird, too! Double click to highlight, and Track Package will be in the right click context menu - and it will open a browser tab with the info!
If you're not sure how to add this to Thunderbird: After you "install" this to Firefox, do yourself a favor and save "link as" to your desktop, and then go to Thunderbird "Tools>>Extensions" and click the install button. Point it to the file on your desktop, and there you have it.