Sandboxie 5.31.6 x64 on Windows 10 1909, FF 70.0.1 x64, similar issues I have reported previously in windows 7 and windows 10 1809. Firefox safemode does not fix this... resetting firefox does not fix this, deleting sandbox and starting with a default profile does not fix this. It always happens after loading a number of tabs at once, say 4-12 tabs of www.whatreallyhappened.com... that recreated it for me. Other times it just starts randomly. Setting Firefox Sandbox to 0 does not fix this. Running firefox outside of sandboxie fixes this.
Are you opening the same tabs and using Firefox for the same duration when unsandboxed? If yes, what is your CPU usage unsandboxed?
I did everything possible to try recreate this problem. Even loading a portable version of firefox in the same (or new) sandbox causes the same problem, even if I don't load any tabs it will often start after about 30 seconds with no warning, almost like on a timer.
Without sandboxie I see 0.1 - 5% cpu browsing, hd movies 8-10%, only short fluctuations above this. In sandboxie, once it is triggered, even if you have one empty tab it will consume 25% of the CPU's electricity constantly.
I've ditched sandboxie for firefox until this is fixed
I get 0% CPU at idle with latest FF and Sandboxie with Windows 10 1809. The only other thing I would have pointed to is an addon that's conflicting with SB but I assume safemode disables the addons. You might consider disabling your AV and anything else that you might have running in the background.
PS: Why does this forum break up words with hyphens? "ad-don"?
Just realized I didn't have any tabs open when I checked my CPU usage. So with 11 tabs open and a YT video playing at 720P I get between 5 to 6% CPU usage while sandboxed.