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Netscape doesn't distinguish between Alt_L and Alt_R in some strange way -- for example if you do xmodmap -e 'add mod4 = Mode_switch' then all Netscape's "left Alt + <key>" combinations stop working, even though Mode_switch (i.e. AltGR/Alt_R) has nothing to do with Alt_L.
My apologies; this is nothing to do with Netscape in particular. Xemacs alerted me to the fact that if you want to use an Alt key as something other than an Alt, you must change its keysym -- it's not sufficient to simply change where it is in the modifier map. Similarly for Control keysyms. So, you can safely discard this "bug" ...