I believe they do use just as much RAM as current tabs, it’s just computers are better at handling it now.
Mozilla makes reference to them eating RAM, but I’m not 100% sure.
Use fewer tabs
Each tab requires Firefox to store a web page in memory. If you frequently have more than 100 tabs open, consider using a more lightweight mechanism to keep track of pages to read and things to do, such as:
Bookmarks. Hint: “Bookmark All Tabs” will bookmark a set of tabs.
Save web pages for later with Pocket for Firefox.
To-do list applications.
I believe they do use just as much RAM as current tabs, it’s just computers are better at handling it now.
Mozilla makes reference to them eating RAM, but I’m not 100% sure.
https://support.mozilla.org/en-US/kb/firefox-uses-too-much-memory-or-cpu-resources