Me too. And my iMac is brand new, intel 10.6.6 OS

nice to know others have the same problem....now to see if anybody has a "solution" to the problem. :)

My "solution":

This is risky because after killing the Firefox 3.6, a successful installation of Firefox 4 is not guaranteed. This may also make the functions of printing and saving webpages as pdf file not working.

1. Backup everything of bookmarks, history, password, etc., by making an extra backup copy of the "Profile Folder" of your firefox. (You may need to turn off Firefox beforehand.) (Not sure how to do this? See notes below.)
2. Make a memo list of all my add-ons and plug-ins. It's a precaution, in case add-ons and plug-ins get lost.
3. Close all applications.
4. Quit my iAlertU in the menu bar. Uncheck "start iAlertU at login" in its preferences.
5. Update everything in my VirusBarrier X6.
6. Use Mac's Disk Utility to repair permissions.
7. Restart my Mac.
8. Login into my administrator account.
9. Go to application folder. Directly delete (command+delete) firefox.
10. Double click the "Firefox 4.0.dmg" in my download folder.
11. Follow the install instructions.
12. Set my iAlerrtU back to its original setting.
13. Try to use Firefox 4. Make sure it works.
14. Restart my mac.
15. Try to use Firefox 4 again. Make sure it works.
16. Clear my recycle bin. (And thus delete forever my old Firefox 3.6.x.)

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If your new Firefox 4 has problem to print or to save webpages as pdf files, this post has some solutions:

http://support.mozilla.com/en-US/questions/684867

At least one solution of them involves killing profile folder, and thus Firefox will lost all bookmarks, history, passwords, and even add-ons. This is why, in steps 1 and 2, I need to make an extra backup copy of the Firefox profile folder and to make memo lists of all my add-ons and plug-ins, before doing the installation.

The backup copy of the profile folder can help you get your lost information back.

The memo list of all the old add-ons and plug-ins, can help you verify anything lost (or not), and, if needed, get it installed again.

The firefox support info about...

Baking up your information: Backing up your information

What a profile is and how to find it: Profiles

Baking up and restoring only bookmarks: Backing up and restoring bookmarks

A mozillaZine.com troubleshooting for Firefox profile: http://kb.mozillazine.org/Profile_in_use

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I'm actually unsure iAlertU, VirusBarrier X6 or repairing permissions to be the source of problem or not. I just got this felling that maybe I could try the step 4, 5, 6, and 12.

During the installation of Firefox 4, it checked the compatibility of my add-on. Some were found not compatible with Firefox 4 and were immediately stopped using.

I forget whether or not the installation checked compatibility of my plug-in.

I don't know whether or not this way would loss any bookmarks or history. I don't remember what I had with my Firefox 3.6.

My mac is MacBook Air 3,2 with OS X 10.6.7. I don't remember exactly which Firefox 3.6.x version I had. The Firefox 4 is of course the 4.0 version.

I use traditional Chinese for my OS X language settings and software versions anywhere available. If not available, I use English. (So please forgive me if any of above technical terms being weird.)

Neither I'm really a software professional, nor I'm enthusiastic. So, to be honest, I don't know what the problem really is or what the perfect, harmless way to install Firefox 4 should be. I just want to try because they said this version is much faster. Maybe I'm just lucky, so good luck to you all.

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It's easier than that. In order to install 4.0, I changed the name of the firefox app in the applications folder to Firefox3.6. Then I moved the new Firefox app into the folder and voila: it opened with all my data intact. I then deleted Firefox3.6.