First things first, you going to need to disable javascript (just to download the xpi files from the Mozilla website).


Once this is complete, go to the extension section of mozilla.org, find the extension you want to download (for the sake of an example, I picked https://addons.mozilla.org/en-US/firefox/addon/1887 ). Right click on the “download now” box and pick “save link as”.



Save the file somewhere.


Next, open this file using WinRAR.


In the xpi file there should be a file called “install.rdf”, open this with notepad (or similar).


Look in this file for a bit of code that looks like “<em:maxVersion>3.0b4pre</em:maxVersion>”, give or take.

Now change the value of this to something like “<em:maxVersion>3.0b9</em:maxVersion>”,

Now save this document, you should get a message like below. Click “yes”.


Now drag and drop file into the extension’s part of Firefox.


Now all you need to do is click install Enjoy!

Thank you Rogem good post = Karma

I just found another way on lifehacker:
Firefox 3 Beta: Make Your Extensions Work with the Firefox 3 Beta

This way is a little simpler.

Nightly Tester Tools has been updated to work with the FF beta now. be sure to Google for the original one, and not the Lite version on Mozilla's site. NTT will force all extensions to be compatable, regardless of the version.

Thank for your information,
Now I use Firefox/2.0.0.14
It is the old version.