Using Firefox's memory snapshot tool, Dominic Tarr measured the heap usage of a variety of web apps. The results are... not good. The biggest losers are Google properties followed closely by Slack (which is probably not a surprise). Fairing much better were GitHub (7.41MB), StackOverflow (2.55MB), and Wikipedia (1.73MB).
What struck me most is that while modern Gmail is one of the worst offenders (158MB), vintage Gmail uses just 0.81MB of memory. The 'good ole' days' strike back!