Have you got the range at the top set to "Everything"? Have you got Browsing and Download History checked?

I did not select "everything" but"Last hour", "Last two hours" or "Last four hours" instead. "Browsing and Download History" is checked

Everything will literally clear out everything, not just the last hour.

ok, I will try out, but in my previous version of Firefox, this feature used to work fine. Thanks anyway

Had the same problem after I updated from FF 4 to 6.

You can (temporarily) set the pref browser.sessionstore.max_tabs_undo to 0 to clear the "Recently Closed Tabs" list.

If you want to re-use the "Recently Closed Tabs" feature (Undo Closed Tabs) then you can reset the pref browser.sessionstore.max_tabs_undo to the default value via the right-click context menu.

• http://kb.mozillazine.org/browser.sessionstore.max_tabs_undo
• http://kb.mozillazine.org/browser.sessionstore.max_windows_undo

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.