Private  +   Shared  =  RAM used Program 
    205.6 MiB +  34.3 MiB = 239.8 MiB firefox-bin

That is with 4 tabs open, one of them being Slack and one being HN. 68.x ESR. Calculated with ps_mem.py [1]

[edit] ran gc / cc in about:memory to get real numbers minus cache. This is on CentOS 7 using the 5.5.6 kernel from ElRepo [2].

[1] - https://raw.githubusercontent.com/pixelb/ps_mem/master/ps_me...

[2] - https://elrepo.org/tiki/About