The reason I recommend Chrome to all my non-techie friends is due to its automatic upgrades. I can be reasonably sure that my friend would be using a modern secure browser at all times.
Firefox's prompts are a pain in the neck. "Do you want to install the latest Firefox?" No! I want to browse the web. That's why I opened my browser. Every time I get that message I groan because I know I should upgrade but I don't want to sit through a download, install and browser restart.
I always thought a "upgrade Firefox on next restart" checkbox which downloaded in the background and kicked off the installer on browser shutdown would be nice.
Meh to prompts. They are a usability nightmare -- they come at the exact time they are most inconvenient: when I am using the browser. And for 99% of people the answer should always be yes. You should have to specifically opt in to prompting and the default should be silent automatic updates.
Every time Chrome auto-updates itself, I notice it because it exposes some new "fit and finish" bug.
The update to Chrome 7, for example, broke the Bookmark Manager if you have your font size increased - e.g. due to eyesight issues. The update to 7.0.517.44 (or maybe an earlier version? I have no idea) changed the word boundaries used when selecting text using the keyboard.
