Try Refreshing your Browser:-

• To do this, click the menu button at the top right corner of the Firefox window.

• Click the “Help” option near the bottom of the menu.

• Click the “Troubleshooting Information” option in the Help menu.

• Click the “Refresh Firefox” button at the top right corner of the Troubleshooting Information page.

• You’ll be warned that this will remove your browser add-ons, reset your customizations, and revert other browser data to the default settings. Click “Refresh Firefox” to confirm you want to make these changes.

Could you open the about:processes page and see how it matches up to the Task Manager? Can you account for all of the processes or are there mystery processes?

Thank you, i did not know about about:processes!

One of the process in about:processes reports 350 Mo of ram while the same PID shows 250 Mo on Windows' task manager. Every other processes report the same ram usage in firefox and task manager.

EDIT: though my firefox just crashed after I took the screenshot (happens when it takes so much ram) - so right now after a restart and 3 tabs loaded it "only" takes 1 gb of ram. I'll make sure to monitor about:processes next time it starts eating my whole ram again.

You can also check the Firefox Task Manager (about:performance) and about:memory pages.

• https://support.mozilla.org/en-US/kb/task-manager-tabs-or-extensions-are-slowing-firefox
• https://support.mozilla.org/en-US/kb/firefox-uses-too-much-memory-or-cpu-resources