Firefox 4 extensions won't install on Mac OS X

Bill Detwiler overcomes two problems installing Firefox 4--Cisco AnyConnect stops the Firefox installation and corrupt files prevent extensions from installing.

Like millions of others, I downloaded Firefox 4 this past Tuesday. I've been using Mozilla's browser for years, and it's still my favorite--despite Microsoft's significant improvements in IE and Google's release of Chrome.

Unfortunately, this Firefox update didn't go as smoothly as the previous ones. First, the application wouldn't install. Then, I couldn't get any extensions to install. Despite scouring the Internet and Mozilla's support site for several hours, I wasn't able to find a definitive solution. My searching did however, uncover several clues that eventually lead me to the answer. Here's what happened.

Firefox 4 won't install

I was running Firefox 3 (with all the latest updates) on an Intel-based MacBook Pro running OS 10.5.8. I downloaded and mounted the Firefox 4.0.dmg image file from Mozilla's website.

Firefox 4 Installation - Mac OS X

When prompted, I attempt to drag the Firefox application folder ( from the image to the Applications folder. The process started, but the following warning message appeared:

Firefox installation libsmime3.dylib error

As this file was located within my current Firefox application folder, which I was replacing anyway, I quickly tossed the file into the Trash bin. Unfortunately, I was greeted with at least two more warning messages about similar files. At that point, I chucked the whole folder in the Trash.

But, when I went to empty the Trash, I was again told that the offending files were in use.

I figured a reboot was in order. Why? Since the files were no longer in their original location, whatever application was using them wouldn't likely be able to do so when I restarted the machine. Luckily this tricked worked. After rebooting the machine and logging into my account, I emptied the Trash and installed Firefox 4 without a hitch.

Note: It wasn't until after I successfully installed Firefox 4, that I discovered my Cisco AnyConnect VPN client was the application holding these files open--even though it wasn't running at the time. While my file-deletion solution worked, I could also have stopped the VPN agent with the terminal command:
sudo killall vpnagentd

Extensions won't install

Then, the real problem appeared. I couldn't install any extensions--one of my favorite things about Firefox. (I won't use Firefox without NoScript, BetterPrivacy, DownThemAll!, and Page Saver Basic.)

I could locate and download the extensions with Firefox's built-in Add-ons Manager, but the installation process would hang every time. After scouring the Web and Mozilla's Firefox Support site, I came and article that suggested deleting the following files:

  • extensions.ini
  • extensions.sqlite
  • extensions.sqlite-journal
  • compatibility.ini

These files are located under the /Library/Application Support/Firefox/Profiles/[random].default folder.

But even after deleting these files and restarting Firefox, I couldn't install my extensions. I then noticed that a folder named "extensions" also existed within the [random].default folder, and it seemed to contain information on lots of old extensions. Figuring the information was corrupt, I moved the whole thing to the Trash and empty the bin.

Voilà! When I restarted Firefox 4, I was able to download and install all my favorite extensions.