Open the about:telemetry link in Firefox and click scalars tab from the sidebar menu. Alternatively, opening about:telemetry#scalars-tab_search=tab will take you directly to the scalars tab.
The browser.engagement.max_concurrent_tab_count key will show the number of tabs active for the session, but does not update when a tab is closed. Instead, if you want to update this value you will need to restart your browser.
The browser.engagement.tab_open_event_count key shows the current number of open tabs at a given time and is updated dynamically.