Unable to install Firefox add-on: when everything else fails

Let’s assume that you are running Vista and trying to install a Firefox extension/add-on like firebug, yslow or zotero. No matter what you do, the “freaking” extensions/add-on will not install. You tried all the recommendations by Mozilla’s article “Unable to install addons“. You Googled for solutions, and even tried a fresh installation of Firefox; still no solution. Why not try running Firefox “as administrator”, by right clicking on the shortcut and selecting “Run as Administrator”. That worked for me. Why?

If you go to %APPDATA%/Roaming/Mozilla/Firefox/Profiles/[random text followed by .default]/extensions.log and see something similar, to what is below then the run as administrator may work.

2009-07-13 17:38:50 – safeInstallOperation: install operation (caller-supplied callback) failed, rolling back file moves and aborting installation.
2009-07-13 17:38:50 – ExtensionManager:_finishOperations – failure, catching exception – lineno: 1658 – file: undefined – [Exception... "Component returned failure code: 0x80520015 (NS_ERROR_FILE_ACCESS_DENIED) [nsIZipReader.extract]"  nsresult: "0×80520015 (NS_ERROR_FILE_ACCESS_DENIED)"  location: "JS frame :: file:///C:/Program%20Files%20(x86)/Mozilla%20Firefox/components/nsExtensionManager.js :: extractExtensionFiles :: line 1658"  data: no]
2009-07-13 17:38:51 – ExtensionManager:_finishOperations – failure, catching exception – lineno: 4969 – file: file:///C:/Program%20Files%20(x86)/Mozilla%20Firefox/components/nsExtensionManager.js – TypeError: entries[i] is null

Note the “NS_ERROR_FILE_ACCESS_DENIED”. Apparently some component on the process doesn’t have the right to extract the files from the xpi and put them on the respective folder (in):

%APPDATA%/Roaming/Mozilla/Firefox/Profiles/[random text followed by .default]/extensions/

Credit where credit is due

Inspiration to this came thanks to this forum thread, although the solution by MANDY (almost at the end of the thread). His solution is the following

I downloaded the actual firebug.xpi file and extracted it’s contents (using 7-zip). I placed those contents into a folder called firebug@software.joehewitt.com.
Then, I went ahead and tried to install firebug as normal. I kept the profiles/extensions folder open so I can see what is going on.
During the install process, as I watch the extensions folder, I see the firebug@software.joehewitt.com being automatically created. Then wait a few more seconds and watch as it disappears.
It is at this point that I paste in the folder that I extracted the .xpi files to.
Back to firefox. Exit. Restart. Firebug works.

You can also open the content of the .xpi file with WINRAR. Just run it and then go to File->Open Archive and you will be able to see the contents of the .xpi. However if you Run as Administrator all this is not necessary and especially when you have to do it for 3 extensions or more.

You can tell me if it works?