Make Firefox 1.5 work with 1.0 Extensions

Martin Brinkmann
Dec 2, 2005
Updated • Apr 30, 2013

Seems to be the time of the year where not only the nights but also the titles of my blog posts become longer and longer. I found something useful for all of you. I had the problem that some of my beloved Firefox extensions would not work with Firefox 1.5 until they were updated by the extension programmer.

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
  3. Enter “app.extensions.version (without quotes) for the preference name.
  4. Then, enter “1.0 (without quotes) as the value for app.extensions.version.
  5. Restart Firefox 1.5, then enable those disabled Firefox extensions.
  6. 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.

Update 2: The latest versions of the Firefox browser assume that add-ons are compatible with it automatically so that it is no longer necessary to use the Compatibility Reporter or other means to force compatibility.


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  1. Administrator said on December 4, 2005 at 11:08 am

    all the extensions i tried did work, which was it that did not work ben ?

  2. Ben said on December 4, 2005 at 4:44 am

    I don’t recommend trying this. I did, and I got DLL errors with the extension I enabled. Firefox would open but it wouldn’t run correctly, and other extensions wouldn’t work. I had to go and reset the about:config entry, uninstall the extension, and reinstall Firefox 1.5 to get it all working again.

  3. Administrator said on December 2, 2005 at 7:41 pm

    I´am not sure. I suppose it could be that some firefox extensions won´t work even though you changed this, maybe because firefox changed how it handels extensions.

    I can´t tell but I think it won´t hurt much, if you encounter a problem simply undo the changes and everything should be back to normal..

  4. FilSchiesty said on December 2, 2005 at 7:29 pm

    I wonder if that would cause any other unforseen problems wiht the fox.

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