That is not the original implementation. The code you see there was an aborted attempt to specify and implement JS2. Brendan Eich wasn't involved, Waldemar is now at Google, and Netscape burned up somewhere in there.
The original implementation was written in C by Brendan Eich, and shipped in Netscape 2. The code currently used in Mozilla Firefox is a direct descendant of that.
It doesn't really look like the original implementation - the dates on all the files are from late 2005, and the credit is to Waldemar Horwat and not Brendan Eich. I'm guessing this was code for prototyping some JavaScript improvement that went into Mozilla recently.
I got the link from http://bc.tech.coop/blog/030920.html which got this description from a LtU blog (now dead). I figured if I didn't know any better, keeping the original description would be best.
And looking at it, I'm still not sure it isn't the original (or rather, early descendant of). Late 2005 is only the license change; the first revision I see (in this folder) is 1999-01-29, and the first version of some files says "initial checkin for source reorg" (remember CVS?).
The original implementation was written in C by Brendan Eich, and shipped in Netscape 2. The code currently used in Mozilla Firefox is a direct descendant of that.