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.

I got an unexpected result:

For context

  pgrep --full /usr/lib/firefox/firefox | wc -l

returns 10, while

  pgrep 'firefox|Web' | wc -l

returns 9.

Your posted command formatted and a missing $ added (I'm using bash):

  echo 1024\*\
  $(\
    pgrep --full /usr/lib/firefox/firefox \
    | xargs pmap -x \
    | grep total \
    | awk '{print $5}'\
    | paste -sd+ - \
  )\
  | bc \
  | xargs ./human

returned 220.63 MiB.

Here's my htop excerpt:

  3532M  286M 97512 S  0.0  9.6  9:20.59 │  └─ /usr/lib/firefox/firefox
  2325M 74708 56596 S  0.0  2.4  0:00.42 │     ├─ /usr/lib/firefox/firefox -contentproc -childID 41
  2509M  121M 85300 S  0.0  4.1  0:15.84 │     ├─ /usr/lib/firefox/firefox -contentproc -childID 40
  2489M  173M  103M S  0.0  5.8  0:18.64 │     ├─ /usr/lib/firefox/firefox -contentproc -childID 39
  2389M 74172 57060 S  0.0  2.4  0:13.49 │     ├─ /usr/lib/firefox/firefox -contentproc -childID 23
  2540M  112M 89996 S  0.0  3.8  0:50.95 │     ├─ /usr/lib/firefox/firefox -contentproc -childID 22
  2513M 72396 58300 S  0.0  2.4  0:14.88 │     ├─ /usr/lib/firefox/firefox -contentproc -childID 17
  2413M 46488 34892 S  0.0  1.5  0:18.37 │     ├─ /usr/lib/firefox/firefox -contentproc -childID 16
   250M  2132  1916 S  0.0  0.1  0:00.13 │     ├─ /usr/lib/firefox/firefox -contentproc -parentBuil
  26.4G 85844 35008 S  0.0  2.8  0:57.39 │     └─ /usr/lib/firefox/firefox -contentproc -childID 3 

Not sure what to make of it.

To capture the other processes pgrep 'firefox|Web' to capture WebExtensions and Web Content

That's what I'm talking about in the beginning of the post. 'firefox|Web' captures 1 one less process than `--full /usr/lib/firefox/firefox`. That one process is 0.1mb in my case.

I didn't know about --full you are correct

I've noticed using some extensions (like Grammarly doubles or triples the amount of RAM used).