I like your attention to those shortcuts though I think we have similar standards:)
If destructive behavior is a bit too "easy" in an app, I remap the command to something complex. I think my Quit in Firefox for instance is now command-shift-control-Q or something.
Though I have to wonder how much resources are taken by the undo feature.
Really? Not just tabs. The danger is you have 5 tabs open, you only want one of them but you accidentally close all of them, now you've lost the window with the tab you wanted.
I have both of these browsers configured to save my sessions and start from where I left off when I quit the application entirely. Both of them persist my closed tab/window history between sessions as well, so I can for example close my way out of 15 tabs across 2 windows, then reboot my laptop, then when I start my browser it will reopen whatever tabs/windows I didn't explicitly close and if I want to restore the ones I closed before the reboot I can use the restore shortcut(s) to get them back.
My only gripe so far is that when I click command+w, I want it to default to closing the window when I am on the blank search page (i.e. where the browser goes by default when you open up a new window), but it only seems to close the window if you have a page loaded.
EDIT: This isn't a big deal. You can close the window in that case with command+shift+w, but the behavior feels inconsistent to me (that the window closes with just command+w when a page is loaded, but otherwise you need shift).