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.

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:

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.