Christophe, in the message you were replying to, I said that I already knew about the hacks that are available to run Venkman. I have it installed on three of my machines already.
There are two problems with it, though: you can only run it once per session (if you close Venkman, you have to restart Firefox in order to open it again), and you shouldn't have to install a hacked version of a core tool like venkman.
Sure would be nice if one of the most important extensions ever written could be updated to work with Firefox 1.5: venkman! There are hacks on the net for making it work, but that shouldn't be necessary for anything as important as the javascript debugger.