Ok, I've been taking a closer look at the problem with SharePoint, modal windows, Firefox and Safari.
Turns out the problem is NOT Firefox/Safari, rather, it is an issue with the setModalDialogObjectReturnValue() function in core.js. Basically, because of differences between Firefox/Safari and Internet Explorer, the result of the 'if' clause in this method is different. For Firefox/Safari, this has the result that the wrong method is used to set the return value of the window.
Here is a very quick and dirty workaround that works in FF3, Safari 3 and IE7 (and WSS 3.0 SP1/MOSS 2007 SP1).
Add the bold line to ...\12\TEMPLATE\LAYOUTS\1033\CORE.JS:
function setModalDialogReturnValue(wnd, returnValue)
if (wnd.opener !=null &&
wnd.opener.document.getElementById('__spPickerHasReturnValue') !=null &&
I put this here only to illustrate the problem and the solution. I have not tested this with older browsers - this code will probably break functionality with older browsers! Preferably, one should modify the 'if' clauses in this method. Also, mind that changes to this file is not supported by Microsoft. It might be overwritten by future Service Packs or upgrades, so you better hope that this bug is fixed in the next Service Pack, or somehow put this fix in some other file.
So who is to blaim? Who should fix it?
What Mozilla changed to make this problem arise in Firefox 3 was simply to add support for the IE specific method showModalDialog(). They didn't have to - they already had another way to do the exact same thing.
No one is to "blaim" - Firefox and Safari supports everything IE supports, and Microsoft made an effort to support Firefox in SharePoint, but even though SharePoint worked in all browsers available during its release, it now fails to properly detect modern browsers.
While it's possible to work around the problem, and it should be quite easy to make the fix as a custom SharePoint solution, I recommend anyone being troubled by this issue to open a support case with Microsoft.