make it working in FF 4.0.1 Rated 4 out of 5 stars
To make this add-on working for FF 4.0.1, I followed next steps:
-search map C:\Documents and Settings\ for file "install.rdf" with the string "formsaver" in it.
-edit this file with a plain text editor: change the line:<em:maxVersion>3.6.*</em:maxVersion> into: <em:maxVersion>4.0.1</em:maxVersion>
Now it is working again. For your own risk, because there might be some incompatible issues with FF 4.0 which should be fixed by the developer.