Today I installed Firefox 4 on 3 computers: 2 windows desktops and 1 MacBook. On both desktops there aren't any probs, on my MacBook pro I can't install any add ons and my 'bladwijzers' aren't displayed as it should.

Create a new profile as a test to check if your current profile is causing the problems. See: * [https://support.mozilla.com/kb/Basic+Troubleshooting#w_8-make-a-new-profile Basic Troubleshooting: Make a new profile] If that new profile works then you can transfer some files from the old profile to that new profile (be careful not to copy corrupted files) See: * http://kb.mozillazine.org/Transferring_data_to_a_new_profile_-_Firefox