After upgrading to M3-436 the UI seems to be behaving strangely.
Initial load of the UI shows "Show Preview Pane" as the main drop down selection for "View" (it should not be there, it should list "Message" and "Conversation"). This problem fixes itself after a bit of fooling around. (but only for that session.).
Some (appearing to be HTML messages) are not loading when I double click on them. Initially (before the "preview pane" problem is fixed) clicking on messages adds a "sash" control to the screen, kindof right under the Zimbra logo. After the other problem has been fixed, it drills down appropriately to the "Message View". However clicking on HTML messages in Message view loads a blank email with only the "External images have not been displayed" dialog (this button does nothing).
I don't appear to have any errors in zimbra.log, /v/l/messages or /v/l/secure. I haven't gotten any complaints from any other users, so this may not be reproducable on windows. I have reproduced it under FF1.5 on Ubuntu Linux (two separate machines).