I did clear the cache (shutdown firefox and deleted the cache dir) and restarted zimbra, with no luck.
updated to firefox-1.0.7-1.2.fc4 in fedora, still no luck
Then downloaded firefox 1.5.0.1 for linux and now I can see the whole mails.
Firefox 1.0.7 on windows -> works
Firefox 1.0.7 from fc4 -> don't works
Firefox 1.5.0.1 (tarball) on linux -> works
Firefox 1.0.5 on windows -> works
So it looks like a bug in the firefox/mozilla package from Fedora Core 4, but strange that it always worked till I upgraded to the zcs 3 GA version.
Thanks for this great software,
Stefan