What would happen if I press "Refresh Tor Browser" on about:support page?
Would the bundled extensions (like Torbutton) be gone too and become standard Firefox browser?
Yes (I once tried it), so DO NOT DO THAT! It will completely remove everything, including Torbutton.
If you want a clean fresh re-install of the Tor Browser just download it again from the official site (https://torproject.org) and delete the previous copy of the Tor Browser and install it again.