Hello, and Welcome to Linux Mint. You can safely remove apps you're sure you don't need, as well as any meta packages that depend on them. The meta packages are effectively just a list of packages installed by default, so removing thunderbird is likely to remove mint-meta-gnome and mint-meta-main which is ok.
If you have a question that has been answered and solved, then please edit your original post and put a [SOLVED] at the end of your subject header
Hint - use a google search including the search term site:forums.linuxmint.com