I'm running 4.5.5_GA_838 (2007-05-11) and very long emails do not completely display using Firefox 188.8.131.52. They do display properly in IE 6.0, which we try to not use. I tried Firefox 3.0 and I can't even login to Zimbra with this latest version.
With Firefox 184.108.40.206, after roughly 2000 lines, text disappears and a black colour fills the panel where the text should be. I realize that 2000 lines is a long email, but we subscribe to some long lists which contain as much as 4000 lines. I have discovered a bit of a workaround, which is to right click the email and choose "Show Original". The only issue with this is that some lines do not wrap, depending on their original format. I can of course paste the content into a text editor which wraps, but these steps are complex and irritating for many of our users.
Any fixes or suggestions?