Since startup time is important, why don't browsers just stay running but pause all threads? The memory consumption of a browser process without any tabs open is insignificant for the vast majority of users.

Google Chrome does this, I think by default. It starts the browser process when you log in to your computer, so when you open a browser window it is really fast.

I just checked Chrome Beta on Ubuntu, Windows and OSX, and there are no Chrome processes running upon login.

No Chrome processes until I actually launch Chrome.

Can you verify?

it is a tick in the settings. something called "Continue running background apps.." in the settings-->advanced -- useful if you need google hangouts running in the background, or to get notifications from websites. I disable that though on all of my machines.