"... While memory usage has improved significantly in recent years, complaints about it have not stopped. If you browse sites like Reddit for example, you still find user's complaining about the memory hog Firefox today.Source: gHacks Tech News
"Here are tips to analyze the issue in Firefox
"The first thing you should do is run Firefox without add-ons and customizations. Each add-on or extension you install may add to the browser's memory usage. Some add-ons, like Adblock Plus for example, may use more memory than the browser itself. ..."
What you do when Firefox uses too much memory