User Agent: Mozilla/5.0 (Windows NT 6.3; WOW64; rv:39.0) Gecko/20100101 Firefox/39.0 Build ID: 20150604162752 Steps to reproduce: On a profile used for a few months (daily, heavy browsing): - Open Library -> History - Right click on a month -> Delete (Reproducible on stable FF38 running on 32-bit Win7, Phenom II X4 945, 2GB RAM, Seagate 7200RPM HDD and FF39 running on 64-bit Win8.1, Core i7 2630QM, 6GB RAM, SSD. Happens with both months and deleting entire sites if they were commonly visited, at least a few thousand entries.) Actual results: Entire browser hangs for about 20 seconds (on a SSD, Core i7-2630QM, fully updated Windows 8.1, nothing hogging the CPU in the background), starts to read a lot of data from HDD (Process Hacker reports 1.6-1.9GB/s), little dialog shown upon r-clicking the month stays on the top until the browser recovers, once it does any downloads in progress fail. Chosen month is successfully deleted from history. Expected results: Browser doesn't do any of that. Chosen month is successfully deleted from history.

I think it's basically the same as bug 871908