The name sort of implies it will interact with my active, open browser, instead, it opens a whole seperate instance of Firefox. This is annoying, and makes the app not better than just using the original individual apps, which at least open inside my open browser.

Вставте Viber

unusefull
It will be usefull if open a single page with all messenger interfaces active into.

Cada que abro TweetDeck me pide Log, y ya abira ahora se queda enblanco y me vuelve a poner el Log, el Messenger me lo abre en la ventana del explorador

Its basically a panel with a bunch of bookmarks. Not sure how this helps anyone except maybe the developer. I for one didn't want a new window opened for every chat client just to be able to use it.

This is NOT a universal messenger. It's just a bunch of bookmarks which open the corresponding web app in a separate window.
Not that this can't be useful for some people, it's just not what it says it is. The description also mentions Slack, while there is no Slack bookmark to be found...

Thanks for wasting my time!