The pop-out window is always considered ‘active’ and this is not unique to Firefox. There is a feature request related here, and this has been discussed here also in regards to Chrome.
Best practice is to ensure that you close the pop out window once you are finished using it.
The extension window closing when it loses focus is also expected - this in itself is a security feature, ensuring that you do not leave the extension ‘active’ causing your vault to remain unlocked - this is essentially addressing the security risk you have highlighted in regards to the pop-out window.
This blog post covers two methods when creating new items within the browser extension - in my opinion the first option is the best way to create a new login item, as it ensures the password is generated by Bitwarden’s password generator. This will help you avoid the data loss situation, ensuring the item is saved to the Bitwarden cloud before the extension window is closed.
Your post does highlight a challenge around measuring what is an ‘active’ session - if the vault timeout period was enforced regardless of window focus, then it presents another issue where the vault will lock immediately once it’s vault timeout threshold is reached, and result in data loss if you were halfway through adding a new item to your vault.