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missing .dll error messages

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First: nsAppShell: Event Window firefox.exe missing xpcom.dll;

Second: Procedure entry point ?JS_ValueToFunction@@YAPAVJSFunction@@PAUJSContext@@VValue@JS@@@Z could not be located

there might have been one more. Browser appears to work once these are bypassed, but I'd like to resolve them to avoid future troubles.

All Replies (6)

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Certain Firefox problems can be solved by performing a Clean reinstall. This means you remove Firefox program files and then reinstall Firefox.

This will NOT delete bookmarks, history, saved passwords, and addons

Please follow these steps:

Note: You might want to print these steps or view them in another browser.

  1. Download the latest Desktop version of Firefox from 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).
  3. Delete the Firefox installation folder, which is located in one of these locations, by default:
    • Windows:
      • C:\Program Files\Mozilla Firefox
      • C:\Program Files (x86)\Mozilla Firefox
    • Mac: Delete Firefox from the Applications folder.
    • Linux: If you installed Firefox with the distro-based package manager, you should use the same way to uninstall it - see Install Firefox on Linux. If you downloaded and installed the binary package from the Firefox download page, simply remove the folder firefox in your home directory.
  4. 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.

More information about reinstalling Firefox can be found here.

WARNING: Do not run Firefox's uninstaller or use a third party remover as part of this process, because that could permanently delete your Firefox data, including but not limited to, extensions, cache, cookies, bookmarks, personal settings and saved passwords. These cannot be recovered unless they have been backed up to an external device!

Please report back to see if this helped you!

Thank you.

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Yes, thank you. I rebooted the PC after following your instructions, and got none of the error messages. I'll save the instructions in case some .dll's get lost in the future.

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Note that is this case it wouldn't be a missing DLL because current Firefox releases no longer use this xpcom.dll file, so you may have other (outdated) software that was still expecting this DLL to be used.

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Hi there, Clean Firefox Install without uninstalling didn't work.

Going back to my apparent solution: Set Firefox to run in Windows Vista (Service Pack 2) Compatibility Mode for all users.

Firefox version 27.0.1 on Windows 7 Home Premium x64.

Modified by Bamboozle

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My apparent solution: Set Firefox to run in Windows Vista (Service Pack 2) Compatibility Mode for all users.

1) Right click on firefox program/shortcut icon to bring up context menu.

2) Select "properties" to bring up "Firefox Properties" dialogue window then select "Compatibility" tab. Click "Change settings for all users" button. A new window "firefox.exe Properties" opens.

3) On the "Compatibility mode" Panel click the tick box next to "Run this program in compatibility mode for:" text box and choose "Windows Vista (Service Pack 2)" in the drop down list box if your OS is Windows 7 Home Premium x64.

(I don't know if this would work for other windows OS versions but it's probably worth a try). If your OS is not Windows 7 Home Premium x64 then try setting the compatibility mode for a previous Windows OS version other than the one installed on your PC.

4) Click "Apply" Button then click the "OK" button then again Click "Apply" Button then click the "OK" button to close the "Firefox Properties" dialogue.

Modified by Bamboozle

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DLL (Dynamic Link Library) is executable files which permits programs for sharing code to perform one or more than one predefined functions. At same time, several programs are able to use DLL files.

So, whenever any DLL file throws any error, it is recommended that you must fix this error as fast you as you can. For fixing the dll errors you will require a professional PC Repair Tool, that automatically detect and fix the related dll errors.

for more info read - What is DLL Error and How to Fix DLL errors?