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CPU usage rises when firefox is left open.
Firefox preforms well when first started but after running for a day or two will have significant problems. Tabs become unresponsive and show only a loading icon in the middle and content processes are stuck at 100% cpu usage. Despite these problems about:performance reports that the worst tabs are using only a few % of cpu usage, just the same as if firefox were started recently.
At first I figured that this was all because I had a lot of tabs in my firefox profile and they used more cpu as they were loaded. Except cpu usage stayed low when I restarted firefox and then made sure to load every tab, only becoming a problem again after a day or so.
Attached are screenshots showing the disparity between about:performance and actual CPU usage and an image of an unresponsive tab.
Additionally, when I quit firefox in this state it takes about a minute after the windows close for the processes to actually exit, and they remain at full cpu usage for that time.
Images would not upload to the support site, so here are some imgur links instead.
Modified by nerwall