In Eclipse, the contents of a tab are represented by an IEditorInput instance which only loads the file (or whatever the editor edits) on demand; if your Eclipse starts with a stored session of 50 tabs, it doesn't need to open 50 files.
The Mozilla team could learn from this.
Meh. I set my IDE to only keep 10 files open at a time. It's such an easy thing to jump into a file nowadays I don't need them all piling up in the background.