1) Disable all plugins and add-ons, and re-enable 1-by-1 until you find the one that leaks. 2) about:memory (will likely show not much as it's almost certainly a plugin/add-on that leaks) 3) Make sure that General->Advanced->Submit performance data is enabled, so if it's really a leak in Firefox, it can be tracked down.

If you still see 1.6G memory usage even with all add-ons and plugins disabled, be sure to report a bug. (I'm assuming you don't have hundreds of tabs open or anything which would otherwise explain that)