Sometimes, after a while, firefox (36.0.4) uses up 1 core of my CPU (is a quad core due to hypertreading). The way it happens is really strange. Many tabs without a problem until suddenly ff decides to floor it. This is alway in the run, not on startup or close. All add-ons need to ask my permissions to be activated, this includes flash. On close, I ordered ff to clear all history and cookies. Notes that could be important (The behavior encountered while using it. This varies from time to time): opening new tabs (a lot or a few) => "heavy" load during site loading, then idling ==> The above makes sense - same as above but load continues to be heavy. => could be some badly written javascript on the site, however, sometimes this occurs with sites I know have no stressful javascript ==> The above seems odd - Browser is idle in the background (not actively using it). Then all of the sudden "lets floor it until closed". ==> The above seems pretty odd - When confronted with high CPU usage, closing ff doesn't close it (the ff process remains active. Not a ff process, THE ff process). => High CPU usage remains ==> The above looks like that there is definitely something wrong here This last one has some other things to keep in mind - When it happens and I close ff, upon restarting it it will ALWAYS ask to close ff. After confirming to close it, it will start and behave normally. - When killing ff via the task-manager, or other means like powershell commands, restarting it will result in an idle ff even after every tab is loaded again. I hope this isn't a heisenbug and there is a solution to this, a reboot ff button would be a nice workaround though. I personally think that it must be some routine in the code that decides to go nuts for no reason. If you need further details, feel free to ask. Thanks in advance