In general, yes more tabs = more RAM used, but Firefox does have a neat trick compared to Chrome that helps lower memory usage for those of us with hundreds of tabs. When you launch Chrome with a bunch of tabs open from a previous session, it actually loads them all into RAM at launch, with Firefox, it doesn’t actually load the pages of tabs from previous sessions, until you switch to them. The page titles and icons get loaded into RAM, obviously, but if you have lots of old tabs that you almost never open, the memory usage impact of lots of tabs is minimized.