Hi   ! Both AdBlock Plus and Norton Security Toolbar use a lot of RAM. Instead of AdBlock Plus you might consider uBlock Origin : https://addons.mozilla.org/en-US/firefox/addon/ublock-origin/ As for Norton Security Toolbar : You're pretty safe with Windows' Defender and a safe browser like Firefox, but that's my personal opinion. Also see : http://tech.firstpost.com/news-analysis/ex-mozilla-firefox-developer-says-anti-virus-software-does-more-harm-than-good-359798.html ---------------------------------------------------------------------------------------------------------- Another reason could be Multi-process (e10s) : If you are using e10s, you could turn it off as follows : Type in the address bar '''about:config''' (press Enter) (promise to be careful, if asked) Type and search for the preferences : '''browser.tabs.remote.autostart''' and set it's value to '''false''' (default) '''browser.tabs.remote.autostart.2''' and set it's value to '''false''' Note: the exact name of the preference may vary, but it will start with browser.tabs.remote.autostart You can check whether you have this feature turned on as follows : 3-bar menu button => ' ? ' button => Troubleshooting Information In the first table on the page, check the row for 'Multiprocess Windows' and see whether the number on the left side of the fraction is greater than zero. If so, you are using e10s - otherwise it should read : 0/1 (Disabled). You could also try this preference : '''dom.ipc.processCount''' And set its value to '1' (if it's set to a higher value)