A co-worker of mine has created a very useful firefox extension for capturing interactions in Firefox. By itself this is useful for scripting common things you do with the web that are tedious yet you do them often. (I had an old coworker who had carpal tunnels who used a similar tool on ie to script anything that he did more than once...).

But then there is the testing functionality. You can save/load scripts as well as replay them in the browser. The scripts are serialized as xml and the project also includes translators to convert these scripts to httpunit (java) and simpletest (php) so they can be run in an automated fashion. (An adapter for selenium could probably be written too...). Perhaps if I get some time (read motivation), I'll try to write an adapter for twill.

The release is in alpha and requires recent builds of firefox (1.5), but is already useful in it's current state.