Nothing. Like there's nothing wrong with Flatpak either.

However, the Flatpak/Snap version of Firefox have some issues. Especially with some browser extensions that need to communicate with tools/apps installed on your system. Their sandboxed nature prevents them from doing so.

For example, a browser extension for a password manager won't be able to communicate with your local password vault app. A video downloader extension won't be able to use the download tool or access the folders you want. Etc.

And the problem here is that in Ubuntu flavors, Firefox is exclusively available as a Snap package from Ubuntu's official repos. So you can't install a .deb. You have to add the external mozilla repos to your sources to do so.

This becomes a bit too technical for most users who are not very tech literate.