Firefox is based off of Mozilla 1.7.2. You can notice 4 major trends below:

1. Firefox is substantially worse today than 1.7.2
2. Firefox keeps getting substantially worse
3. SeaMonkey has become substantially better than Mozilla 1.7.2
4. SeaMonkey is -- by far -- the most efficient major browser available today.

* Open 500 wikipedia pages -- in tabs if possible
* Close all but one
* Reopen all again
* Close all but one

Total downloaded size: 1.95 MB in compressed text and images.

Firefox 1.5 -- aka Deer Park Alpha 2
Initial: 31 MB
High: 584 MB
Low: 470 MB

Firefox 1.0.6
Initial: 32 MB
High: 392 MB
Low: 369 MB

SeaMonkey 1.0a
Initial: 18 MB
High: 89 MB
Low: 30 MB

Mozilla 1.7.2
Initial: 32 MB
High: 139 MB
Low: 69 MB

IE 6 -- Only 38 pages could be opened before new windows were frozen
Inital: 22 MB
High: 120 MB
Low: 28 MB

Opera 8.02
Initial: 14 MB
High: 130 MB
Low: 102 MB

Firefox is a great idea. A horrible implementation.