Recently, I installed Firefox 4 (codename: Minefield and version: 4.0b13pre) in my pc. I am amazed to see its new look and features. But started feeling bad when some of my useful add-ons were not installed. Actually Firefox 4 does not allow to install these incompatible add-ons. So, I had to do a little hack on it ion order to make those add-ons comparable in firefox 4.
First, I opened a new tab and typed: about:config
A pop up message: I’ll Be Careful, I Promise came. I just accepted it
Then right-clicked and selected New, then Boolean
Another pop up menu came where I typed the preference name according to my version
Then set the value to false
Finally, I closed the tab and restarted my Minefield. After that I did not have any problem to install my desire add-on.