It hasn't done that in years. I have ~20 tabs open and about 500 MB of memory used. For comparison chrome uses the same amount of memory at about 10 tabs open.
Just had to killall firefox. So I beg to differ. They just blame the third party addons. Like ABP, because it's your fault for installing one of the most popular plugins.
Fortunately there's an easy way to get it. Can you visit about:memory when memory usage gets high, click on the "Measure" button and post the results here, or email me, or file a bug at bugzilla.mozilla.org and put "[MemShrink]" in the "whiteboard" field? Thank you.