Lean URL has a few serious shortcomings. I've fixed those in my fork called Neat URL.

https://addons.mozilla.org/firefox/addon/neat-url/

The improvements in Neat URL:

* set your own URL parameters on the options page (to reach feature parity with Pure URL)
* fixed for recent Firefox versions
* a nice animation in the toolbar (can be changed or disabled)
* domain-specific blocked parameters (to reach feature parity with Pure URL)