Why do you think it's dumb that Messenger is through Facebook? It is the messaging system of Facebook, after all. You don't need to use a facebook.com URL, if that's what you mean - messenger.com exists. But of course it's the same system under the hood.
For messages (text and attachments), I have no trouble using Firefox - I just keep a tab open with messenger.com in it. And for privacy I use an extension called "Facebook Container" that utilizes Firefox's container tabs feature to keep all Meta connections (even APIs) separate from everything else I do online. Works like a charm. The only problem is video/audio calling - the newest Messenger encryption mechanism for calls doesn't support Firefox, so I have to use an app for that. But I am absolutely not leaving Firefox.