We're here to help! This relates to the Firefox browser. You can close the sidebar at anytime by pressing the 'X' in the top right, or clicking on the sidebar icon in the Firefox UI which will prevent it from opening in a new window. Contact the support team directly at https://bitwarden.com/contact/ for additional assistance.

Thanks for the support and feedback! That doesn't sound like expected behaviour, please contact our support team with more details at https://bitwarden.com/contact/ or file a bug report on https://github.com/bitwarden/clients/issues