Why htop show firefox use more mem than I actuall do?

[leafonsword]

As you could see,htop show many firefox threads,each use about 20% of total mem,but totally firefox will use mem more than actually mem,how could this happen?

[liquiddandruff]

Htop is currently showing each thread as a process. To change this, go to htop settings > display options > hide userland threads, or press shift+h.

[leafonsword]

@liquiddandruff Thanks～