Change your location bar to include "History and Bookmarks".

You could also try changing your location bar to Bookmarks or Nothing, but I have not tried that so I don't know if it would solve the problem.

Unfortunately, changing the location bar to "History and Bookmarks" did not help. When I checked the box "Clear history when Firefox closes" and then closed Firefox, the process ran amok again and I had to manually kill it.

But I do have some additional information. After recovering from the test by resetting Firefox twice, I changed my settings to "Never remember history" (without checking the box to "Clear history..."). I was able to stop and restart Firefox, and the process stopped and started normally. I noticed the link telling me that I might want to clear my history. I clicked on it, and once again, my CPU usage spiked, along with the I/O reads, and the process got stuck again. The disk access light, however, did not flicker. So presumably, whatever Firefox is repeatedly reading was in the disk cache.

It appears to me that Firefox is unable to clear the history, but will not give up trying. Perhaps there is some exception which is being handled, the routine which tries to clear the history never reports success but cannot cause an abort, and the software is looping inside some routine with no exit. Robust, but erroneous.

Trial-and-error attempts with the privacy settings are extremely time-consuming, because every time this happens, I have to re-enter the list of allowed and blocked domains. It would be nice if the reset method could remember these along with the cookies.

So sorry that did not work for you.

If I come up with anything else, I'll let you know by posting it here.

Are you clearing the cookies?

That can be caused by clearing the cookies via "Clear Recent History".

• https://support.mozilla.org/kb/Clear+Recent+History
• Firefox/Tools > Options > Privacy > Firefox will: "Use custom settings for history" > [X] Clear history when Firefox closes > Settings

If you use "Clear Recent History" to clear the cookies then Firefox also tries to clear the cookies from plugins like Flash (LSO) by opening plugin-container processes.

You can let all cookies expire when you close Firefox.

• Firefox/Tools > Options > Privacy > Cookies: Keep until: I close Firefox

Create a cookie 'allow' exception for cookies that you would like to keep.

• Firefox/Tools > Options > Privacy > Cookies: Exceptions

Note that clearing "Site Preferences" clears all exceptions for cookies, images, pop-up windows, software installation, and passwords.

Clearing cookies will remove all specified (selected) cookies including cookies with an allow exception that you would like to keep.

No, I actually want to retain the cookies, considering that I only have cookies from domains which I have explicitly authorized. When I attempted to enable "Clear recent history when Firefox closes", I made sure that the box to clear cookies was not checked.

All I wanted to do was simply try out the feature. It's not actually that big of a deal for me, but after seeing the film Don Jon... well, you'd hate to ruin a perfectly good affair with Scarlet Johansson just because she didn't like something in your browser history :-)

I had the runaway process occur again on closing when I removed two Firefox extensions and clicked the link to restart. But unlike when I checked the box to clear the history, after killing the process I didn't have any trouble restarting (i.e. no need for a reset). I tried to replicate that behavior by reinstalling and re-removing the two extensions, but it did not happen again.

This leads me to conjecture that the root of the problem might be a deadlock between some threads/processes. As a J2EE developer, I run into database deadlocks on occasion, and I know that in a multi-threaded app they are notoriously difficult to track down.