Steps to reproduce:
Open any website
Click the three dots to open the menu
Click “Add-ons”
Click “Bitwarden”
Enter your master password
From that moment threre is no any way to close the add-on screen but close the app.
How to proceed with that?
Hi @l4gfcm, welcome to Community! 
The Firefox add-on for Android is not officially supported. We recommend using the Bitwarden mobile app for the best experience!
Got it, thanks!
So why is this extension even in the tightly curated Mozilla add-ons library for Android? And is the regular Android app fully tested to be on par with site fills in the Firefox browser? Part of the problem for me over the years has been the inconsistent nature of web login fills and the browser extension would at times work when the app wouldn’t. Supposedly everything fully supports the Android autofill services framework now, but I still end up hitting sites that won’t trigger any sort of autofill options in the keyboard. It’s very hit or miss.
You can close the add-on screen, but you have to press the “back” button a bunch of times. This is due to Firefox Android no longer having the ability to directly close an extension popup since the transition to Fenix from Fennec.
My speculation is that this is due to the popup frame having session history stack entries that are normally unnoticeable on Firefox desktop, and you have to back through all of those before the popup is closed by backing out (via the browser chrome’s back button for the popup) of the initial entry of the session history stack, but I haven’t verified this via debugging yet. The source is hard for me to statically analyze, but I think the following sources confirm my suspicion: org.mozilla.fenix.addons.WebExtensionActionPopupFragment subclasses org.mozilla.fenix.addons.AddonPopupBaseFragment, which has override fun onBackPressed(): Boolean, part of an implementation for interface mozilla.components.support.base.feature.UserInteractionHandler, which I think ends up being used by mozilla.components.support.base.feature.ViewBoundFeatureWrapper. (The sibling code from Reference Browser might be easier to follow than Fenix’s; start from org.mozilla.reference.browser.addons.WebExtensionActionPopupActivity (note how it directly checks for windowRequest.type == WindowRequest.Type.CLOSE in override fun onWindowRequest(windowRequest: WindowRequest)).)
Until Mozilla resolves this lack of an actual WebExtension popup close button (I couldn’t find a relevant bug on Bugzilla—see bug list for quick search “ALL platform:All|Android OR component:Android OR product:Fenix :WebExtensions popup”), the Bitwarden popup could add a close button (possibly only on Android) that calls window.close(), which should still be supported by Fenix’s GeckoView (support was added in https://bugzilla.mozilla.org/show_bug.cgi?id=1612363). (I’ll leave it up to your engineers to file the feature request bug for this missing close button if they like. I do think it’d be generally useful.)
@sj-bitwarden I do hope that the Firefox Android WebExtension becomes officially supported; the integration is nice enough that I’ve been happily using it alongside the native Bitwarden Android app for years.