Early March 2014 problem arose in Thunderbird. E-mail sent to me from Android phone disappeared. Went to ISP and restored e-mail. All windows decorations disappeared! Every application! All programs started in upper left corner of screen. No ability to move windows at all. Re-boot had no change in this effect. Wine internal WM worked OK. Applications menu worked, so I used Firefox to find how to install decorations in a different WM. New WM worked OK, but I like Xfwm4 theme. I used a terminal with just "xfwm4 --replace" and it worked once, thereafter it did not. I used terminal with "sudo xfwm4 --replace" and WM came on with decorations. I used ^c to break and decorations disappeared. I then used "sudo xfwm4 --replace &" and decorations re-appeared. Somewhere in startup Xfwm4/Ubuntu 12.04 there must be a "decorations" element specification switch that I can turn on from a terminal but is not turned on during initialization by Ubuntu. The replacement WM set the switch for itself correctly, but fixing the Xfwm4 switch is beyond me. Can anyone help?
In Settings -> Session and Startup -> Session tab, make sure xfwm4 is present and set as "Immediately" in the "Restart style" column. Then save your session and it should be fixed in subsequent sessions.
Thanks for your suggestion.
Unfortunately, the problem could not be resolved from the menus. I tried that early on. Problem seemed as if the start information was read wrong. What did work was to re-install the entire xfce WM.
When this was done, a new problem appeared: The WiFi no longer connected automatically. I have to open a menu os "hidden networks" and select the one in my house. This is a minor inconvenience, so that is how I have left it. I did modify the xfce startup to ask me which session I might want. This allowed me to select a session that worked the previous time.
I have since installed Ubuntu 14.04 KXStudio on a separate partition that previously held Ubuntu 11.04 KXStudio as my main working O/S. My son suggested that I have a separate partition for Thunderbird shared among Ubuntu 12.04, Ubuntu 14.04, and Win-7. This works very well, and several programs I have on Ubuntu 12.04 remain there as I use them infrequently and do not want to port them.