Mozilla doesnt detect long URLs. You can see this behaviour in the thread "Lange URLs und 76 Zeichen Längenbegrenzung" in de.comm.software.newsreader. http://bugzilla.mozilla.org/show_bug.cgi?id=106405 is probably related.
Using 1.0RC1, I get a nice question prompt asking whether I would like to subscribe to 4ax.com... no errors. Please consider updating to the latest release if you haven't yet. Since this bug was filed almost five months ago, I will assume that it will work for you now with a newer release (since you and I use the same O/S) and resolve the bug. However, if you are still having a problem with this, feel more than free to reopen the bug and to provide additional information. Thanks for testing Mozilla.
I subscribed to the newsgroup mentioned above. The thread mentioned is no longer available. I did go through the other messages to in that newsgroup for long url's they are all OK. reporter if you see this again and reopen this bug please put an example of the length of the url that's not detected. If the url listed in the original scenario is an example of a long url then it's been fixed.
Fortunately my newsserver (news.btx.dtag.de) still carried the postings. Please see http://bugzilla.mozilla.org/attachment.cgi?id=84812&action=view as an example.