You should in general prefer to install from the official Manjaro or AUR repos whenever possible. But in the rare case (cough Microsoft Edge cough) where this is not possible, here are a sequence of steps you can follow to install a .deb package in Manjaro (and other Arch based distros)
You need to install yay. Once that is done, install debtap using yay:
yay -S debtap
Before using debtap you need to let it build the package database at least once, so run this command once:
sudo debtap -U
Now convert the deb package to an Arch package:
debtap <your deb>.deb
This will create the Arch package. Now install that using your package manager (pacman for example):
sudo pacman -U <your arch package>
This easy, helpful procedure was gathered from this answer in the Manjaro forum.
You need to check this:
sudo debtap -u and not sudo debtap -U
sudo debtap .deb and not debtab .deb
You need to check this as well:
debtap .deb not debtab