Rated 5 out of 5 stars
It's easy...here's how:
1. Right click on the "Add to Firefox" download button and select "Save Link As..."
2. In the dialogue box that comes up, select the Desktop to save it to.
3. Find the addon-190-latest.xpi file on your desktop and change the file extension to .zip (When you get the warning asking if you're sure you want to change it, click on "Yes".
4. Extract the zip file.
5. In the extracted zip folder find the file called install.rdf. Open the .txt file with Notepad
6. Find the following text:
<!-- Firefox -->
7. Change the maxVersion from 3.6 to 4.0 (should look like: <em:maxVersion>4.0.*</em:maxVersion>)
8. Click on File and Save
9. Close the text file
10. Close the extracted window
11. Change the auto_copy.zip file back to .xpi
12. Drag the auto_copy xpi file onto an open Firefox 4.0 window which will install it