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Andavari is correct.

Nergal, I mostly use CCleaner to delete cookies and the browser cache. I DON'T mess with the registry, and that is NOT what I wrote. Your comments are not relevant.

Exactly. As to exactly caused the crash, I will probably never know either (unless Mozilla can tell me from their crash reports). Checking task manager doesn't help as much as it used to. A recent change, in windows 10, seems to place the "most used" program/ component at the top of the list and that listing order changes about every second, making it almost impossible to read! Unless someone has found the cause and verified it, any other replies are useless.

I have NOT had a problem with Firefox taking an excessive time to close under NORMAL use. I have run CCleaner almost immediately after closing Firefox and never had to wait for the past several years. Firefox "closing unexpectedly" is NOT normal use!

I understand that Firefox has to be closed to run CCleaner. Normally this is not a problem. Today (October 09, 2019) Firefox unexpectedly closed while I was checking my Yahoo mail. I re-opened it and I finished, then closed Firefox. When I went to remove some cookies with CCleaner, I was informed that Firefox was still running, when it had been Closed and there was NO indication it was still running with (Windows 10) task manager. I opened Firefox (currently 69.0.2) then closed it twice before trying to remove some cookies with CCleaner. When I went to delete those cookies, I was informed that Firefox was still running. CCleaner opened a window that offered to terminate Firefox. I clicked the option to close Firefox. A few seconds later, I was told that Firefox was not closing and was offered an option to force it closed. I clicked that option and only then was I able to delete those cookies I wanted gone. What does CCleaner use to determine that Firefox is NOT running? Does Firefox closing unexpectedly freeze this, whatever, to indicate that it did not close? Whose problem this is I don't know, but I have also informed Mozilla that there is a conflict with CCleaner when Firefox closes unexpectedly. Problem solved, until the next time Firefox closes unexpectedly. I use Firefox almost exclusively.

My windows 7 computer has three accounts. Two regular user accounts and an administrator account. I am administrator, but use one of the user accounts for web browsing. I attempted to update CCleaner (free version) after logging on to the administrator account. Downloading was fine, but when I went to install version 5.04.5151 (64 bit) it gave me the error: "CCleaner v5.04 setup error opening file for writing". According to your FAQs, this error says I need administrator permission. I WAS logged in AS Administrator. I gave permission for the program to make changes to my computer. It STILL gave me the error! I switched to my regular user account, and downloaded the file. I told it to run the file. I was asked for the administrator password, and entered it. the program installed with NO PROBLEMS! Is this a problem with CCleaner or Windows 7? Windows 7 Pro (64 bit) up-to-date as of April 06, 2015. ( SP-1 and ALL non-optional updates installed)