Cumulative CPU means the total amount of CPU used by the addon or tab, since measuring began.
This might be useful to measure the long-term impact of addons or tabs which aren't always busy, and so might not be caught easily by watching the about:performance tab.
Actually, the reason I want this is that I suspect one of my addons causes Firefox to slow down during startup, but then goes quiet after startup. I would like to find out which one it is!
Therefore, whilst a checkbox on the about:performance page to toggle cumulative recording would be good, I would also like:
- Recording of the data even when about:performance is not open, and
- a setting (e.g. in about:config) which would last between restarts, or
- a flag on the command-line which would turn the option on.
(If people want a similar feature for memory, I suppose it might be useful to display the peak memory usage for each addon/tab, rather than the current memory usage.)