Shareaza does not really interact directly with Firefox, it might however interact with a plug-in. So I'd check installed plug-ins and check if disabling them did something.
Also if you think there might be something wrong with Firefox, doing a full reinstall of it might help. I've had some issues with Firefox myself where the updater does not work fully, ie a bug that should have been fixed a few versions ago still persists after Firefox has been updated. So far doing a full manual install has always worked. No idea what the issue is/issues are.
And if the network connections Firefox uses are reset before they complete the transfers as some network filtering systems actually do when they detect P2P traffic, it would make the rendering engine unable to render correctly. Or rather not rendering the page until you have downloaded it without getting a reset is probably the correct way; the reset is supposed to mean the server has some issue serving the file, which implies it should not be used (it might be invalid) and you should try again.