For the most part, add-ons (extensions and themes) are trouble free. However, once in a while an add-on will not install, upgrade or uninstall correctly. Or may be you launched a different version of Firefox on the same profile and now your add-ons are disabled because Firefox says they are incompatible.I will cover the three most common issues with Add-ons Installation:

• Software Installation Disabled
• Extension(s) not installed after restarting Firefox
• Add-on(s) Disabled by Firefox

Software Installation Disabled
This issue mainly occurs when upgrading to a major version of Firefox such as going from 0.* to 1.0.* or 1.0.* to 1.5.0.* versions. For some reason Firefox disabled the installation of themes plug-ins and extensions XPI files. These instructions for re-enabling downloads, are from the Mozilla site:

Firefox 1.0

1. From the ‘Tools’ Menu select ‘Options…”
2. Click ‘Web Features’
3. Place a check in the box ‘Allow Sites To Install Software’

Firefox 1.5
Starting in Firefox 1.5 the option to allow web sites to install software has been removed from the user interface (UI). The software installation option must be enabled to allow installation of themes, extensions and certain plugins

If you previously disabled this option and then updated to Firefox 1.5, the disabled setting will be carried over. Since the option to enable software installation no longer exists in the UI, you will need to use about:config to reset the “xpinstall.enabled” preference to the default “true” value. Here’s step by step how to do this:

1. Type about:config into the Location Bar and hit enter.
2. Type xpi to filter and find xpinstall.enabled.
3. Make sure it’s value is true. (double click will toggle).
   

Extension(s) Not Installed After Restarting Firefox
This issue is becoming less common but use to occur frequently with extensions that did not upgrade correctly. There were some extensions that required you to remove them before installing the new version. Failure to do so resulted in a phantom extension that could not be upgraded or removed.

If after restarting Firefox the extension has not installed go to the Tools Menu and select ‘Extensions’ (FF 2.0, select ‘Add-ons’). If the extension appears in the list with this message ‘(Name) will be installed (removed/upgrade) when Firefox restarts’, then follow these directions:

1. Close Firefox
2. Locate your Profile Folder
3. Delete the following files from your profile folder:
   • extensions.ini
   • extensions.cache
   • extensions.rdf
4. Restart Firefox.

Firefox will rebuild the extensions list and these files. Your extension should be installed and functioning now.

Add-on(s)Disabled by Firefox
Firefox will automatically disable any extension that is not compatible with build you are running. This will happen if you try to run and older or newer version of Firefox on the same profile. However, even after you return to the correct version of Firefox, it will not automatically re-enable the add-ons for you. This happened to one our Go Firefox! members who posted:

“…I made the mistake of trying out Bon Jovi…That piece of work has now disabled all my extensions on FF 1.5”

To start, it’s Bon Echo after the park in Canada, not after the 80’s New Jersey rock group. Second and foremost, they should not have been trying out this “piece of work” with the same profile as they were using with Firefox 1.5. This very reason is why I stress the importance of running separate profiles for testing in my Profiles Revisited posting.

But…this same thing happened to the humble Guru. I had installed the newer version on Bon Echo and didn’t noticed the box ‘Run Bon Echo after installation is completed’ was checked. It ran on my FF 1.5 profile and all my extensions became disabled, even after re-running 1.5. I ended up having to go through and reinstall (or upgrade) all my extensions (not an easy task when you have 38 extensions). The good news is you shouldn’t have to go through and reinstall all your extensions. In order to restore your add-ons follow these directions:

1. Close Firefox
2. Locate your Profile Folder
3. Delete the following files from your profile folder:
   • extensions.ini
   • extensions.cache
   • extensions.rdf
4. Restart Firefox.

Firefox will rebuild your extensions and themes list and these files. Your extension should be re-enabled and functioning again.

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Note: This post was adapted from FF Extension Guru’s post, Add-ons FAQs: Troubleshooting Add-ons Installation from Go Firefox!