Not going well with Kaspersky Support
CPU problem started for me on Monday 24th, ie date of new FF extension - I have 8 windows with 100ish tabs total that I have to reload with "Restore Previous Session" when I repeatedly have to close and restart FF, running on i5-4690 3.5Ghz with FF Extension version 2.4.33.0.
I reported this problem by chat on Monday May 1st and gave pretty clear description of how disabling and reenabling the Kaspersky FF extension reproducibly changes the CPU use by 35% approx for me. [In Windows TM with the extension enabled I see two serious CPU hogs from FF, one runs at about 25% and the other at 10%]
I was told there is no known bug for this. When I said I'd seen incident reports on this thread I was told that "information was awaited".
I was told to perform some diagnostics with first Exiting, then Pausing protection, then (if problem was stopped by Pausing) selectively disabling stuff. Oddly no warnings about risks of disabling Kaspersky while online, I did my tests with network disconnected. And also oddly no diagnostics of the FF extension requested in the script.
As Pausing the protection did not stop the problem and the diagnostic script said to do no more tests but to advise I did that, by email later, and also added comments to restate the reproducible effect of disabling and reenabling the FF extension.
Oddly I got a reply asking why I hadn't followed the rest of the script. So I have replied to say I followed the instructions. I've not heard back.
If I close a few windows I get less CPU use, so perhaps this is a per-tab cpu use thing and thus not seen by people with a small number of tabs open?
Anyway, it doesn't look like this is being worked on as a bug I suggest more people report it and add any useful information about number of open tabs etc.
Looking in Windows TM I also notice that FF has an "undulating" memory use that varies by several GB over 10s of minutes, I think this was happening before the CPU problem started, but it does seem to go away when the extension is disabled. This also happens even when the network is not connected.