Mozilla with Firefox 10 introduced the compatibility by default feature which enforces compatibility natively for the majority of add-ons.

As a consequence, the compatibility enforcing capabilities have been removed from Add-on Compatibility Reporter. The add-on is now only a reporting tool that provides Mozilla and add-on developers with compatibility information about their add-ons.

Firefox users who have installed the extension solely for the reason of enforcing add-on compatibility, can remove it from the browser as it is no longer offering that functionality.

The compatible by default feature does not enable all add-ons by default. Brian King lists the following exemptions from the rule:

• Add-ons marked to work with a Firefox version less than 4.0
• Add-ons with binary components
• Add-ons explicitly marked by the author as incompatible, i.e. opt-out of Compatible By Default
• Add-ons tested and determined to not be compatible with a given version of Firefox, and marked as incompatible by Mozilla
• Themes

Most add-ons currently installed in Firefox 10 or higher should be automatically enabled by the new compatible by default feature. Users who are still running pre-Firefox 4 add-ons on the other hand that they now are working in Firefox 10 and higher are left standing in the rain.

One option for them to make their add-ons compatibility would be to install the Nightly Tester Tools to force add-on compatibility. Another option would be to edit the Firefox add-on file directly. Check out Three Methods To Force Add-On Compatibility In Firefox for information on how to do that.

Please note that this affects Thunderbird and SeaMonkey as well.

Is It Compatibly is a Firefox add-on that aids the Firefox user in assessing the add-on compatibility of the web browser. All the add-on does is to display the versions of Firefox that are compatible with each add-on in the browser’s add-on listing.

A Firefox user wanting to upgrade the web browser would simply check that listing prior to upgrading the web browser to see if the new version of Firefox is supported by the add-ons. This compatibility check is a semi-automatic process as the user needs to check the versions against the version of Firefox that will be installed on the computer system. This can take a while for users who have dozens of add-ons installed.

Is It Compatible is a helpful add-on for users who want to make sure that their add-ons are compatible with a new version of Firefox. The add-on itself is compatible with all Firefox 3.x versions and can be downloaded from the Mozilla website.

Method 1: Nightly Tester Tools

The Nightly Tester Tools have been designed for Firefox developer’s who want to test (their) add-ons in new versions of the web browser. This Firefox add-on forces compatibility so that the add-on can be installed even if the compatibility information say otherwise. This is a very straightforward and easy way. Firefox users with this add-on usually do need to download the add-on that is not compatible with their version of Firefox from the add-on’s page that is displaying all versions for download (The last version download usually becomes inactive if the Firefox version of the user is not compatible).

The Nightly Tester Tools will display the following window if the add-on has been found to not be compatibly with the version of Firefox:

It is also possible to force compatibility for all add-ons that are already installed in the add-on manager in Firefox.

Nightly Tester Tools is available at the Mozilla Firefox website.

Method 2: Editing the Firefox Configuration

This is the method that has been changed by the developers. It has an impact only on Firefox 3.6 beta 3 and newer which means that most users will not have experienced the changes yet. Before that version it was possible to create a new Boolean parameter extensions.checkCompatibility in the Firefox configuration and set it to false to be able to install all Firefox add-ons regardless of their compatibility.

This was changed by the Firefox developers so that it is now necessary to add a version to the parameter. What this means is that it is from Firefox 3.6 beta 3 on only possible to override compatibility for a specific version of Firefox. The times of changing a parameter once are therefor a thing of the past.

The Firefox user needs to append the version of the browser to the parameter, for Firefox 3.6 beta 3 it would be extensions.checkCompatibility.3.6b, for the final version of Firefox 3.6 extensions.checkCompatibility.3.6 and so on.

The advantage of this method is that it does not require the user to install an add-on to force compatibility. It has become more complicated with this change. It does not on the other hand provide the same level of customization that the Nightly Tester Tools offer (the parameter enables all add-ons automatically and the user would have to disable or uninstall the add-on to stop using it).

Method 3: Editing the Firefox add-on file directly

Now this method was popular when Firefox 1 became more popular. Today it’s more of a nice to know method than a practical one as the other two methods should be all there is needed to make an add-on compatible with the Internet browser. Users need to download the add-on to their computer system first. The add-on is an xpi file which then needs to be extracted with a program like 7-zip. This will extract all the files of the add-on including a file called install.rdf in the root directory.

Open that file in a text editor and locate the the following two lines:

3.0b5
3.7a1pre

These two parameters define the compatibility limits of the add-on. Change the versions (usually the maxVersion is all that needs to be changed) so that the Firefox version is included. After that create a new zip archive with the updated contents and rename the extension from zip to xpi. Install the edited add-on in the web browser by dragging and dropping it into its interface.

It turned out that my favorite RSS add-on Brief for Firefox was not compatible with the release candidate 2. I tried to force compatibility which did not work either. Now here is the interesting information that might help you if you ran into a similar problem. If one of your add-ons is not compatible to the latest Firefox version you could visit the developer’s homepage to see if he did post a new version or a workaround.

It turned out that the developer was busy creating Brief beta versions, the latest on his homepage was Brief 1.2 beta 2 while the latest on the official Mozilla website was Brief 1.1. Brief 1.2 beta 2 is fully compatible to Firefox 3 and I was able to select my feeds and use the RSS Feed add-on ever since.

To sum it up. If you have an add-on that you need but that is not compatible try to force compatibility. If that is not working check the developers homepage for new versions that have not been posted on the official Mozilla website yet. It worked for me.

I had no intention to wait and thought of installing a Firefox 1.0 version next to the new 1.5 but lucky Martin found a site that explains how to make Firefox extensions that do not work with your Firefox version work in your Firefox version. That sounds complicated somehow, lets see how this is done..

1. At the location bar, enter: about:config. This will show you a list of Firefox internal preferences.
2. Right-click on the list, select New > String
Enter “app.extensions.version (without quotes) for the preference name.
3. Then, enter “1.0 (without quotes) as the value for app.extensions.version.
4. Restart Firefox 1.5, then enable those disabled Firefox extensions.
5. Restart Firefox 1.5 again to active the extensions. Done.

That’s it, five easy steps and every extension should be working like before.

Update: Firefox 1.5, that’s is a long time ago. Technologies have changed in the last six years, and it is now easier than ever to force compatibility of your Firefox add-ons with newer versions of the browser. Keep in mind though that forcing compatibility does not really ensure that the add-ons will work in the new version of the browser.

Mozilla furthermore has announced that they will soon make changes to add-on compatibility. This basically means that the browser from then on will assume that add-ons are compatible with the browser, instead of assuming they are not.

The easiest way for now is the Add-on Compatibility Reporter add-on, which enforces compatibility for all Firefox add-ons. Just install the extension and you should see that it will automatically enable all incompatible add-ons.