I'd recommend starting a new profile, add

-profilemanager

after thunderbird.exe on your shortcut, or browse to your thunderbird.exe from the run menu (Windows key + R) and append that suffix before clicking OK, create a new profile folder in your home folder and choose that as the location, name the profile something that will remind you which one is which. When setting up your Gmail account, if you have a lot of messages or a slow connection, you might want to be judicious about which folders to set as being available offline under the account's server settings (and disable Thunderbird's junk filter as Gmail will already have done a good job of that before mail hits your Inbox).