Hi qianli, when you are in about:config, if you click the Status column heading, the custom settings will be grouped together, so that might make it easier to undo your changes. You can right-click > Reset the preferences related to what you might have changed.

If that doesn't work, most settings changes you make in about:config are stored in a single file named prefs.js so if you want to revert all custom settings (your changes plus data Firefox has recorded), you could try the following. Note that this also clears your home page, pinned/blocked tiles on the new tab page, and toolbar layout, so it might be more drastic than you're looking for.

Open your current Firefox settings (AKA Firefox profile) folder using either

• "3-bar" menu button > "?" button > Troubleshooting Information
• (menu bar) Help > Troubleshooting Information
• type or paste about:support in the address bar and press Enter

In the first table on the page, click the "Show Folder" button. This should launch a new window listing various files and folders in Windows Explorer.

Leaving that window open, switch back to Firefox and Exit, either:

• "3-bar" menu button > "power" button
• (menu bar) File > Exit

Pause while Firefox finishes its cleanup, then rename prefs.js to something like prefs.old.

By default, Windows hides the .js file extension. You can force Windows to show all file extensions using the steps in this article: http://windows.microsoft.com/en-us/windows/show-hide-file-name-extensions

Start Firefox back up again and you should have mostly default settings.

If the problem is that the site doesn't recognize the types of files you want to upload as valid (for example, it doesn't recognize PDFs), there may be a different, easier to solve problem.

Open your current Firefox settings (AKA Firefox profile) folder using either

• "3-bar" menu button > "?" button > Troubleshooting Information
• (menu bar) Help > Troubleshooting Information
• type or paste about:support in the address bar and press Enter

In the first table on the page, click the "Show Folder" button. This should launch a new window listing various files and folders in Windows Explorer.

Leaving that window open, switch back to Firefox and Exit, either:

• "3-bar" menu button > "power" button
• (menu bar) File > Exit

Pause while Firefox finishes its cleanup, then rename mimeTypes.rdf to something like mimeTypes.old.

Start Firefox back up again and it should consult the Windows registry to rebuild the associations of content types and file extensions with your system default settings. Any improvement?

The Refresh feature (called "Reset" in older Firefox versions) can fix many issues by restoring Firefox to its factory default state while saving your bookmarks, history, passwords, cookies, and other essential information.

Note: When you use this feature, you will lose any extensions, toolbar customizations, and some preferences. See the Refresh Firefox - reset add-ons and settings article for more information.

To Refresh Firefox:

1. Open the Troubleshooting Information page using one of these methods:
   • Click the menu button , click help and select Troubleshooting Information. A new tab containing your troubleshooting information should open.
   • If you're unable to access the Help menu, type about:support in your address bar to bring up the Troubleshooting Information page.
2. At the top right corner of the page, you should see a button that says "Refresh Firefox" ("Reset Firefox" in older Firefox versions). Click on it.
3. Firefox will close. After the refresh process is completed, Firefox will show a window with the information that is imported.
4. Click Finish and Firefox will reopen.

Did this fix the problem? Please report back to us!