I didn’t notice any changes to tab opening when Firefox activated native Wayland (I’ve been running Wayland full-time for a couple of years now). However, I was finally able to remove a couple of Mozilla Wayland-specific settings from my .bashrc file though:
Regarding tabs not opening in the foreground, I would recommend that you visit about:config in Firefox to access advanced preferences, then enter inbackground in the filter bar, and make sure that browser.tabs.loadDivertedInBackground and browser.tabs.loadInBackground are set to false.
You might also want to install plasma-browser-integration from the Manjaro repos (if not already installed) and add the Plasma Integration addon to Firefox. I don’t think they will impact your issue of tabs not opening in the foreground, but they will make Firefox work better with the Plasma desktop environment.