I have this type of issue more with Chrome than Firefox though and it always seems to be related to leaving a tab open with a video for a long time, even if it isn't being played.
And the about:memory page, which shows you memory usage of all your extensions too.
I have this type of issue more with Chrome than Firefox though and it always seems to be related to leaving a tab open with a video for a long time, even if it isn't being played.