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Couldn't Load XPCOM

Daniel41
Daniel41
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Last night Windows 10 did another forced update and all of my program files residing in "programs (x86)" were deleted. I did a restore and while the program files are back, now when I try to launch Firefox I get the error message "couldn't load XPCOM". Do I need to uninstall/reinstall Firefox and if so what happens to all of my bookmarks? Also do I need to do anything with the registry?

Last night Windows 10 did another forced update and all of my program files residing in "programs (x86)" were deleted. I did a restore and while the program files are back, now when I try to launch Firefox I get the error message "couldn't load XPCOM". Do I need to uninstall/reinstall Firefox and if so what happens to all of my bookmarks? Also do I need to do anything with the registry?

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jscher2000 - Support Volunteer
jscher2000 - Support Volunteer
  • Top 10 Contributor
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First, why not make a backup of your Firefox data. If you paste this shortcut into your Windows 10 search box, it should launch a file viewing window to the folder with your Firefox settings in it: 

%APPDATA%\Mozilla\Firefox\Profiles

Typically you have one folder with a partially randomized name. If you find more than one folder, you probably only need to back up the most recent one. To create a backup, you can right-click > Copy the folder and then find a safe place (away from any Mozilla folder that Firefox might modify) and right-click > Paste it there. Due to some large data files, this might take a minute or two.

Then, download and run a fresh Firefox installer as follows:

Clean Reinstall

We use this name, but it's not about removing your settings, it's about making sure the program files are clean (no inconsistent or alien code files). As described below, this process does not disturb your existing settings. You do not need to uninstall Firefox first.

It only takes a few minutes.

(A) Download a fresh installer for Firefox 49.0.2 from https://www.mozilla.org/firefox/all/ to a convenient location. (Scroll down to your preferred language.) Choose the "Windows" version (32-bit) rather than the 64-bit version for maximum plugin compatibility (and of course for 32-bit Windows systems).

(B) Exit out of Firefox (if applicable).

(C) Using Windows Explorer/My Computer, rename the program folder as follows:

32-bit Firefox on 64-bit Windows: Rename 

C:\Program Files (x86)\Mozilla Firefox

to 

C:\Program Files (x86)\OldFirefox

Other combinations: Rename 

C:\Program Files\Mozilla Firefox

to 

C:\Program Files\OldFirefox

(D) Run the installer you downloaded in step (A). It should automatically connect to your existing settings.

Note: Some plugins may exist only in that OldFirefox folder. If something essential is missing, look in these folders:

  • \OldFirefox\Plugins
  • \OldFirefox\browser\plugins

Back in business?