yes, FF is a memory hog.

im not a FF programmer, but each tab tends creates its on instance

so what you can do is a right-click on the disturbing instance and execute an End Process" to see which website was sucking up memory

It is quite normal to see more Firefox processes running.

• process for the main Firefox thread (user interface)
• one or more content processes, see:
  Options/Preferences -> General -> Performance
  remove checkmark: [ ] "Use recommended performance settings"
• process for the compositor thread
• process for extensions
• process for WebRender when this feature is enabled

You can find the current multi-process state on the Troubleshooting Information page (about:support).

• "Help -> Troubleshooting Information"
  "Application Basics": Multiprocess Windows
  "Remote Processes" further down

See also the about:memory page for information about the process IDs.

You can 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