Security theatre. If your browser is attacked by an 0day of any sort, or malware or whatever, it will have access to the shared credentials and information inside your browser. This means any and all cookies, shared logins, account access, tab information — is all game. That's where all the actual value is; why would I care if it's inside a VM or not when I can get credentials to your mail provider from the browser itself and just exfiltrate? It doesn't matter if you restart the browser once or a million times; anytime it has sensitive information, it is a target. Unless you plan to literally restart/wipe after every interaction with every domain in a separate same-origin policy where any sensitive information exposure occurs.

But if you're that careful, what is the VM really doing for you, and why the hell are you even exposing yourself that much? Just use Lynx or something.

The real solution is this: Install Firefox, install noscript to nuke all javascript, install ublock too, and get a password manager. Selectively allow any webpage interactivity, as necessary. The world isn't a Tom Clancy novel so you don't actually need to do anything more than this to be very secure and on top of almost all active threats.

Ultimately to achieve what we all actually want (strong isolation guarantees that would prevent a full browser exploit from both A) your SSH keys from getting stolen and B) also your gmail spool from being attacked, and let's be honest, B is the worst case scenario) requires a rethinking of the fundamental software stack from OS to user-visible applications. No amount of Browsers-in-a-VM are a substitute.