I have FF 3.0.19 and am trying to install Xform Version 0.8.6ff3. Nothing happens on clicking the link to install it (https://addons.mozilla.org/en-US/firefox/addon/mozilla-xforms/versions/?page=1#version-0.8.6ff3).
First, you need to update off of Firefox 3.0.19. It is extremely old (it was released in 2008) and horribly slow and insecure. Please go to www.getfirefox.com and download Firefox 13.0.1 IMMEDIATELY.
After doing that, unfortunately the xForms add-on has been discontinued. http://www.philipp-wagner.com/blog/2011/07/the-future-of-mozilla-xforms/ it is not supported or updated. xForms in the browser will have to be developed some other way, instead of an extension.