Some Firefox extensions are released on this site. They are free and can be downloaded from the official Mozilla Add-ons website. I would try my best to update them as soon as possible for the latest stable version of Firefox. Some users love these extensions and want to know whether I can upgrade them for the upcoming preview version of Firefox. I do not have a plan to do that yet, but that does not mean you cannot try my extensions in the newer Firefox. In most of the cases, you can do it yourself by following the tip below. It is very simple.
1. On Firefox Add-ons website, go to the description page of the extension you want. If the extension is not for your Firefox version, the "Add to Firefox" button is gray instead of green.
2. Right click the "Add to Firefox" button, and select "Save Link As ..." from the context menu. Save the xxx.xpi file to your harddisk.
3. Unzip the xxx.xpi file with a Zip application, such as WinZip, WinRar, 7-Zip.
4. Among the extracted files, look for a file name install.rdf. Open it with a text editor. Search for the tag "em:maxVersion" in the file. (The file is not big. You can usually just scroll down a little bit to see it.)
5. Change the value of "em:maxVersion" to the version of your Firefox. For example, you may see the original value is "3.6.*", and you can change it to "3.7a5pre" to enable it for the preview of Firefox 3.7.
6. Pack the the files back with the changed install.rdf by the Zip application you use. The zipped file may be named as xxx2.zip. Rename it to xxx2.xpi.
7. Run your Firefox. Click menu "File|Open File..." and open xxx2.xpi. Firefox will ask whether to install this extension you built. Answer yes to install it.
You may also try to change the "em:minVersion" tag in the install.rdf file to support an earlier Firefox version, e.g. from "3.0" to "2.0". But I believe most of the time a new extension does not support the old Firefox.