I have exactly the same problem with exactly the same software installed (OS 10.6.4; trying to install Firefox 3.6.6 over 3.6.3). The web is full of references to the error message "The operation can't be completed because you don't have permission to access some of the items" but none that I can find tell me what to do, at least not in this case with Firefox. The automatic update in Firefox is what tipped me off that there was a problem. It kept throwing up a message that it couldn't update, so I tried manually downloading 3.6.6 and installing it, and I could not do it either.

Hi, I found http://support.mozilla.com/en-US/forum/1/576083 and it said to just manually drag the old firefox.app file from Applications to the Trash and then drag the new firefox.app file to Applications. I did it, and it worked fine. And I lost no bookmarks. Note that I did not use AppCleaner or any other tool (e.g. AppZapper), which probably would have deleted the bookmarks. Problem solved for me. I'm now running Firefox 3.6.6

I deleted the Firefox 6.0 app and installed the 6.6 app. I did not receive the error message: all of my add-ons and bookmarks were kept intact.

Thank you, all.

Thank you for the solution--I had the same issue too and have been trying to install it from time to time over the last month or so--thanks for finally putting my frustration to rest!

As per your suggestion, I dragged the old firefox icon from both my apps folder and my dock into the trash. Then I clicked and dragged the firefox icon into the apps icon via the arrow and it finally worked. Note that instead of "opening" the download initially, which I had done in the past, I had "saved" it to my applications folder in the first instance--however, that did not work by itself. I still followed the procedure recommended here.

Repairing permissions and even manually changing them does not work.

I'm running Mac OS 10.4 (tiger) and in the past got the don't have sufficient permissions and/or can't do it because "contents" is in use error messages.

Thanks again. -Marianne

Thanks for solving the same problem for me before my needing to ask the same question as Bill, above.