Actually, this is a browser incompatibility with Netscape/Mozilla. Certain combinations of servers/phpBB versions produce code (gzip-compressed HTML) that the browser can't understand, thus a default empty page is displayed. Gzip-compressed http transfers are something of a standard and is supported by all modern browsers, furthermore it's only used if the browser sends the "Accept-encoding: gzip" line with the http headers.
So basically, Netscape/Mozilla tells the server that it can handle the compressed data, but in reality it can't. Whether this is the server producing faulty code or the browser having incomplete support I don't know. The fact that IE can parse the code fine seems to support the second conclusion. (then again, IE does quite a lot of things that isn't part of any standard)
The only solution I know to this problem would be to completely deactivate gzip-compression for the entire forums. Unfortunately, the extra bandwidth that would be generated from such a switch is ...well, more than I can handle at the moment. Apart from that, all I can do is keep upgrading phpBB, or if it's possible to allow users to disable gzip manually for their individual account.
