If you have a recent firefox you may be incorrect about your ram usage as firefox is no longer a single process.

Here is how it looks on my system.

        firefox-bin─┬─GPU Process───35*[{GPU Process}]
                    ├─Privileged Cont───27*[{Privileged Cont}]
                    ├─RDD Process───5*[{RDD Process}]
                    ├─Socket Process───5*[{Socket Process}]
                    ├─Web Content───28*[{Web Content}]
                    ├─2*[Web Content───27*[{Web Content}]]
                    ├─Web Content───33*[{Web Content}]
                    ├─Web Content───22*[{Web Content}]
                    ├─Web Content───30*[{Web Content}]
                    ├─Web Content───32*[{Web Content}]
                    ├─WebExtensions───27*[{WebExtensions}]
                    ├─firefox-bin
                    └─97*[{firefox-bin}]

Here is a bit of bash to turn a number of bytes into a human readable label shamelessly stolen from somewhere saved as human

    #!/bin/bash
    b=${1:-0}; d=''; s=0; S=(Bytes {K,M,G,T,P,E,Z,Y}iB)
    while ((b > 1024)); do
        d="$(printf ".%02d" $((b % 1024 * 100 / 1024)))"
        b=$((b / 1024))
        let s++
    done
    echo "$b$d ${S[$s]}"

Here is the best formulation of finding the memory usage of foo where foo can be a regular expression

    echo 1024\*(pgrep foo| xargs pmap -x|grep total|awk '{print $5}'| paste -sd+ -)|bc|xargs human   

In fish the meminfo function with custom functions not herein included

   function meminfo
        pgrep $argv| each pmap -x|g total|nth 5|sumof|multiplyby 1024|each human
   end

Most of firefox's mememory is in firefox, Web Content, and WebExtensions

meminfo firefox returns only 250MB right now but meminfo 'firefox|Web' returns 1.5GB

If you are looking at just firefox you are definitely not getting the whole picture.