Based on mozilla-1.2's configure file, it seems that --enable-extensions=... should be used instead of --with-extensions=... Please find attached a patch which makes this change as well as allows the disabling of the irc extension via a "moznoirc" USE flag.
Created attachment 6100 [details, diff] Patch that changes "--with-extensions" to "--enable-extensions" and allows disabling of irc extension.
Hi, even updated the default extensions, etc, but was seemingly blind in not noticing the flag change =) Fixed on CVS, thanks!.