I would buy into that argument more if Firefox actually released all the memory from tabs when you were done doing using them.
Coincidentally i just happen to have 100 tabs open, spread across 9 windows, and Firefox is currently consuming 2,871,288 K of private memory.
Close one window with 8 (graphically dense tabs). Wait 30 seconds. Now down to 2,802,295 K.
Close a window with 15 tabs of webcomics. Wait 30 seconds. Now down to 2,717,452 K
I won't bore with you with the rest of the details. Continue closing windows, then tabs, until this post is the only tab left. Still using 1,979,024 K!
The other 99 tabs were apparently just a little it's over 9000 K each, but this last tab is holding on to almost 2 GB of memory with a death grip
The incentive to close extraneous tabs and windows is pretty minuscule when it doesn't actually gain me that much. So instead i open as many tabs as i feel like, then just close everything and start over when either Firefox or the PC starts getting sluggish.