It must be a misunderstanding. They probably are not aware of how to trigger the excellent garbage collector shipped with the browser.
Let's navigate heavily a bit, opening several new tabs, click click click around, then closing those tabs:
$ top -b -n 1 | egrep "firefox|PID" PID USER PR NI VIRT RES SHR S %CPU %MEM TIME+ COMMAND 6842 enrico 20 0 882m 398m 17m S 2 19.9 24:29.55 firefox-bin
Indeed Firefox memory usage quickly grew. I've seen it grow to a gigabyte and a half after keeping it open for a month.
Now let's trigger the garbage collector:
$ kill -9 6842 $ # Rerun firefox $ # Click on "Restore previous session" $ # Wait for the various open tabs to reload
And voilà! Now Firefox uses much less memory than before:
$ top -b -n 1 | egrep "firefox|PID" PID USER PR NI VIRT RES SHR S %CPU %MEM TIME+ COMMAND 5242 enrico 20 0 656m 205m 25m S 0 10.3 0:14.49 firefox-bin