I tested it with marketplace messages open. It cycles through ALL the messages in the list from the top (most recent), clicks the hamburger, deletes.
No option to select messages to delete or keep.

Stops after 10 and demands you subscribe to continue. Click the button and it loads a page demanding $5 a month.

Ah, no, kid, I am not going to turn you into a billionaire so I can delete my messages, I will get AI to write me a decent add-on.

Firefox is a community of users who contribute free addons to help the community.

It throws a popup advertising other addons at the bottom of the page (not sure if they are available in FF) and you have to disable the addon to get rid of it.

Messages I had already archived were still in the archived list, but when I unarchived them they did not appear in my main message list. Not sure if this was due to the addon or not.

Don't use this thinking you can re-send a message you wanted to keep: when I tried to send replacement message to replace a deleted message, I got a error "cannot send message". So I can't contact that person anymore. And IDK if they can contact me.