That means that Firefox has crashed. See http://kb.mozillazine.org/Firefox_crashes and Troubleshoot Firefox crashes (closing or quitting unexpectedly)

Firefox 3.5 and later versions will automatically try to restore the tabs and windows from the last session if a crash has occurred or Firefox wasn't closed properly the previous time. If it didn't work the first time and the session crashes another time then you will get the "Well this is embarrassing" page (about:sessionrestore). See https://wiki.mozilla.org/Firefox3.1/AboutSessionrestore_Security_Review

The reply from cor-el is not helpful. The behaviour I observe is as follows: every time Firefox is shut down without a chance to save its state, e.g. if a Windows automatic update reboots the computer, the next time I start Firefox I always get the "having trouble recovering windows and tabs" message. There is no "didn't work the first time and crashes a second time" scenario as described in the reply above. It always fails. However, the "this is embarrassing" page does show all the tabs I had open previously, and it also contains a button labeled "restore session", which always works when I press it. Clearly, Firefox does know how to restore my session successfully, because it can do it whenever I press that button. Why can't it do that without me having to press the button?

What is the value of the pref browser.sessionstore.max_resumed_crashes on the about:config page?

• http://kb.mozillazine.org/browser.sessionstore.max_resumed_crashes

To open the about:config page, type about:config in the location (address) bar and press the "Enter" key, just like you type the url of a website to open a website.
If you see a warning then you can confirm that you want to access that page.

• Use the Filter bar at to top of the about:config page to locate a preference more easily.
• Preferences that have been modified show as bold(user set).
• Preferences can be reset to the default or changed via the right-click context menu.