Rated 4 out of 5 stars
I tried a little experiment and altered the .xpi installer to install on Firefox 4.0. So far it appears to be working and my Clickonce projects launch fine. I take no responsibility for it working or not working but anyone who wants to try, do the following:
1) Download the microsoft_.net_framework_assistant-1.2.1-fx-win.xpi package from this site.
2) Temporarily change the .xpi extension to .zip.
3) Open the package and edit the install.rdf file.
4) Change the version in the "em:version" line from 1.2.1 to 220.127.116.11. This should allow official releases to update the add-on yet make it unique from 1.2.1.
5) Change the "em:maxVersion" line from "3.6.*" to "4.0.*".
6) Save install.rdf back in the .zip file.
7) Rename the .zip back to .xpi.
8) Open the.xpi with Firefox and let it install.