You can check the browser.sessionstore.* prefs on the about:config page and reset user set (bold) browser.sessionstore.*prefs to the default value via the right-click context menu to make sure that Session Restore is enabled and working properly.

• http://kb.mozillazine.org/Session_Restore
• https://support.mozilla.org/kb/restore-previous-session

Make sure that you do not run Firefox in (permanent) Private Browsing mode (Never Remember History).

• https://support.mozilla.org/kb/Private+Browsing

• Tools > Options > Privacy > Firefox will: "Use custom settings for history"
• Deselect: [ ] "Always use private browsing mode"

If you do not keep changes after a restart or otherwise have problems with preferences, see:

• http://kb.mozillazine.org/Preferences_not_saved

Thanks for you reply. I have gone to the about:config page and reset user set (bold) browser.sessionstore.*prefs. But that doesn't help me with the "undo close tab" that appears in a menu when you right click on any tab. I also have a problem with the "recently closed tab" found under history on the menu bar.

I have also reset the defaults on firefox again. Both of those options worked for a short while. But once I turned off the computer and restarted everything again they don't work again.

Are there any prefs changed after you reboot the computer?

You can make a backup copy of the prefs.js file after you close Firefox and see if the file got changed after the reboot.
A compare of the two files will show which prefs got changed.

• http://winmerge.org/

The preferences you set in Firefox can be overridden by add-ons and external software.

• If Firefox works normally during the rest of your browsing session after you reset the preferences, and is not broken again until after you exit and restart the browser (or shut down and restart Windows), I would suspect external software. Perhaps you have other software with a feature of increasing browser privacy? Please see this article (linked in cor-el's post above), especially the part about a possible user.js file: How to fix preferences that won't save.

• If Firefox ignores your preference changes or reverts them immediately, an add-on, particularly an extension, is a more likely culprit. Try disabling ALL nonessential or unrecognized extensions on the Add-ons page. Either:
  • Ctrl+Shift+a
  • orange Firefox button (or Tools menu) > Add-ons

In the left column, click Extensions. Then, if in doubt, disable.

Usually a link will appear above at least one disabled extension to restart Firefox. You can complete your work on the tab and click one of the links as the last step.

I had this problem as well, I couldn't select "Restore recently closed tabs" under History because it was grayed out.

I went to about:config und changed browser.sessionstore.max_tabs_undo to 10 from 0 (which 10 is default), und I set browser.sessionstore.max_windows_undo to 3 from 0 (which 3 is default).

For me, this fixed my issue und now I can restore recently closed tabs.

Hi P5yx0Z0, thanks for posting that solution.

It appears that an add-on or external software might be modifying your settings. If it happens again, could you report on what Firefox extensions you have enabled, and whether you use any external privacy or system cleaning software that might be causing this?

Thanks for everyone's help. I'm not the most tech savvy person, so sometimes it takes me a while to work through all the answers. P5yx0Z0's answer has helped the most.

You are most welcome. I'm glad it helped you out. ^^

jscher2000,

I can assure you it wasn't an add-on.

I actually wiped my computer und installed Windows 7 Professional, everything was stock, downloaded Firefox, und Firefox was installed like that on my computer. It came grayed out. All I did was download/install it from the firefox main page. Opened it up, und started to collect the files I had stored on my cloud service. So nothing specifically was interacting with the way Firefox behaved, because I hadn't gotten that far yet to customize firefox.

However, when I accidentally closed a tab, und I went to restore it, that's when I noticed it was grayed out.

So, it wasn't anything on my end that caused that. At least logic would dictate it wasn't anything on my end that caused that. Because simply downloading files in zip format to put back on my computer from my cloud wouldn't have had an effect on firefox.

It was a simple fix for me, being average at working about:config since I've had to tinker so much within the last few years with it.

I set those two things back to their normalities, as well as a few other thing of my own preference, und Firefox hasn't given me any issue.

Now, all my issues have been long since resolved.

Danke, though. ^^

Edit to add: Also, to clarify, I hadn't installed any software on my computer at the time, since all my stuff was in cloud. I just downloaded everything in zip files. Hadn't even gotten around to installing anything except for Firefox.

I had the same difficulty. Could not use the Ctrl+Shift+T combo to undo close tabs. The solutions mentioned here did not work. I turned off the add-on "Tab Mix Plus" and the keystroke combo worked. Updating to the latest version of Tab Mix Plus solved the issue.