new messenger opens links in EDGE instead of default browser.

I believe Firefox has its own method of creating web apps, and it works fine in Vivaldi (Chromium-based).

I'm sure it does, but the only reason I want an app at all is because Messenger's new way to doing video/audio calling only supports Chrome, Opera, and Edge, according to this page. I was perfectly happen to just keep a tab open with messenger.com in it, until Meta made that change earlier this year. The old version of the app is working well for me - I responded to Martin's post just to clarify for other readers that "your favorite browser" doesn't mean any browser but only Chromium-based ones.

I think Firefox had this feature, but because the Mozilla team are super lame, they dropped it. But you can achieve it with a plugin. I think this one: https://addons.mozilla.org/en-US/firefox/addon/pwas-for-firefox/

I think there might be one called electron too.

In this web centric age, on an app centric OS, this feature is essential IMO. Even Microsoft is too lame to figure this out.

This should open Messenger in your favorite browser...

Only if your favorite browser is Chrome. That doesn't work in Firefox for several reasons, and might not in other non-Chrome browsers either.

im not seeing that option. does it still work?

When you downloaded it, it should have asked if you wanted to pin it to your taskbar. Choosing yes would put it in your taskbar for easy access.