Add a prompt indication that you're in a directory with firebase.json (aka "firebase project").

Download the plugin file firebase.plugin.zsh and move it to ~/.oh-my-zsh/plugins/firebase/firebase.plugin.zsh.

Add the given plugin to your plugins list in the .zshrc file:

plugins=(git firebase)

Update the shell prompt of your theme:

robbyrussel is the default oh-my-zsh theme. Update the PROMPT variable in the file .oh-my-zsh/themes/robbyrussell.zsh-theme with $(firebase_status_robbyrussell):

PROMPT='${ret_status} %{$fg[cyan]%}%c%{$reset_color%} $(firebase_status_robbyrussell) $(git_prompt_info)'

Update the PROMPT variable in the file .oh-my-zsh/themes/eastwood.zsh-theme with $(firebase_status_eastwood):

PROMPT='$(git_custom_status)$(firebase_status_eastwood)%{$fg[cyan]%}[%~% ]%{$reset_color%}%B$%b '