i have the same problem, running mac 10.5.8

ok i trashed the current firefix, and installed the new version. no problems.

When you did a new install you would have had to update all favourites and bookmarks. Is there an easy way of doing this ??

Sorry - name included was autofilled - didn't realise to change it.

it did transfer all the info, bookmarks, etc. didn't empty trash before i knew it took.

See also http://support.mozilla.com/tiki-view_forum_thread.php?comments_parentId=633100&forumId=1 - the problem may be with the sharing and permissions on the Firefox app. Reinstalling is one way to do this; changing the permissions is another.

If the terminal commands are unfamiliar, you should be able to accomplish the same thing by finding the Firefox app, selecting it and hitting Command-I, and in the bottom of the Firefox Info window, set yourself or your group to have Read & Write access to the application.

I have this same problem on Vista with all the later versions of Firefox. I can get around it by closing Firefox and then right clicking on Firefox and choose "run as adminstrator". It's ridiculous that I should have to do this though, Firefox should just go ahead and download the update and then trigger UAC when the user goes to install. If they don't have rights then they won't be able to install. But still it's ridiculous that Firefox does not talk to UAC. Grrrr....

For Windows 7 (may work the same for other versions of windows): Left click the Firefox icon (start menu, desktop, where ever) and select "Run as administrator". The use the Help menu in Firefox to "Check for updates...". Any updates will install without the error message.

It is extremely aggravating that Mozilla has allowed this defect to exist for so long without a proper fix. Regardless, the above process works for me.