Try the Firefox Safe Mode to see how it works there. The Safe Mode is a troubleshooting mode, which disables most add-ons.

(If you're not using it, switch to the Default theme.)

• You can open the Firefox 4.0+ Safe Mode by holding the Shift key when you use the Firefox desktop or Start menu shortcut.
• Or use the Help menu item and click on the Restart with Add-ons Disabled... menu item while Firefox is running.

Don't select anything right now, just use "'Start in Safe Mode" To exit the Firefox Safe Mode, just close Firefox and wait a few seconds before using the Firefox shortcut (without the Shift key) to open it again. If it is good in the Firefox Safe Mode, your problem is probably caused by an extension, and you need to figure out which one.

Please follow the Troubleshoot extensions, themes and hardware acceleration issues to solve common Firefox problems article for that.

When you figure out what's causing your issues, please let us know. It might help other users who have the same problem.

Thanks for that, but I'd already tried Restarting with Add-Ons Disabled. Doing that enables you to close the offending tab, but it doesn't help debug the problem, as it occurs only intermittently.

Any other ideas? Thanks in advance.

You said that the issue is resolved when Firefox is in safe mode - if it works in safe mode, it means that there is an add-on that is causing the issue in which as explained in the first response. Thanks!

Yes, I understand that. But how can I work out what add-on is causing the problem. Because the problem is (apparently) intermittent, adding add-ons back one by one doesn't tell you anything. Is there a logging mode I can go into to establish what caused the problem next time it occurs? Thanks.

One thing you can do is start disabling add-ons one by one and just using it that way. When the issue comes back, disable another add-on. One thing you can always do is a clean reinstall.

Certain Firefox problems can be solved by performing a Clean reinstall. This means you remove Firefox and any leftover program files and then reinstall Firefox. Please follow these steps one by one:

1. Download the latest Desktop version of Firefox from http://www.mozilla.org and save the setup file to your computer.
2. After the download finishes, close all Firefox windows (click Exit from the Firefox or File menu) and confirm all upcoming messages.
3. Now, uninstall Firefox by following the steps mentioned in the Uninstall Firefox article.

IMPORTANT: On Windows, the uninstaller has the option to remove your personal data and settings. Make sure that you do not check this option; otherwise all of your bookmarks, passwords, extensions, user customizations and other Firefox user profile data will be removed from your computer.

After uninstalling Firefox on Windows, delete the "Mozilla Firefox" program folder, located by default in one of these locations:

• (On 32-bit Windows) C:\Program Files\Mozilla Firefox
• (On 64-bit Windows) C:\Program Files (x86)\Mozilla Firefox

1. Go to the Windows Start menu and click on "Computer".
2. In the Explorer window that opens, double click Local Disk (C:) to open the C:\ drive.
3. Find the "Program Files (x86)" folder or "Program Files" folder.
   • On 32-bit Windows, double-click the Program Files folder to open it.
   • On 64-bit Windows, you will see a "Program Files (x86)" folder AND a "Program Files" folder. Open the Program Files (x86) folder.
4. Look for a Mozilla Firefox folder. If you find one, right-click it and select Delete and confirm that you want to move the folder to the Recycle Bin.

Now, go ahead and reinstall Firefox:

1. Double-click the downloaded installation file and go through the steps of the installation wizard.
2. Once the wizard is finished, choose to directly open Firefox after clicking the Finish button.

Please report back to see if this helped you!