First of all you are recommended to remove the older version of firefox(probably 3.x). In synaptic package manager search for firefox and mark the box for removal.Wait for few minutes until the package is removed; then follow the steps given below -
step #1 : Open Synaptic Package Manager and add the Firefox PPA to the software resources. Go to Settings->Repositories->Other Software and click on Add button; enter this ppa address -
step #2 : Click on reload button to update the software Repository information.
step #3 : Now,search for ‘firefox 4′ and mark the check box for installation.
step #4 : Next, click on Apply button to begin installation. Then wait for the installation to complete.That’s all!From: Firefox 4 for ubuntu 10.10