I think it's only turned off because it's the default, and for most usages - including 99.9% of Hebrew, which won't be pointed with vowels - the default renderer suffices.

I've now got Mozilla 1.7.3 installed from Fedora Core 3. about:configure shows Pango is enabled. I've got the font set to one which can handle Hebrew vowels correctly. And yet it's still rendering it wrong. *sigh* Should have known it looked too easy a solution. :-S