FWIW - I have encountered this only one time(as of now), on ParrotSec OS (I know it's debian, I believe it is stretch) after installing firejail and running thunderbird firejail'd (pluma was also open at the time, as was the Mate panel editor and a couple folders). The computer slowing down was caused by process kswapd0, and the process hogging all the RAM was mate-settings-d. Also, the computer didn't become wholly unresponsive, but it was close. For instance, the mouse arrow moved very, very slowly, and no clicks or other commands would register until kswapd0 got done. I closed out all firejail'd applications and killed the mate-settings-d process in the terminal and the computer runs fine, no other steps needed (although it did take a minute for the kill command to take effect. Once it did, though, the computer was immediately back to normal). Thing is, re-running thunderbird, firejail'd or not, now has no effect. No warnings, no slowdown, no nothing. Doesn't seem to matter what else is open.
I know that isn't a lot of technical information, but it is contextual. I'm told every little bit helps.
