One thing that could help is showing what is going wrong. Do just the icon does not appear? Do some error show up?

But regardless, I see that Librewolf is not packaged in Debian official software repositories (online storage a software packages are downloaded from), so they ask you to add their own repository manually, which for APT case (package manager in Linux Mint) is an overwhelming amount of code to type to say at least.

You say you are a new user, so I can highly recommend that if something is not officially available through simple apt install to try Flatpak. Official guide: https://flathub.org/setup/Debian, TLDR:

sudo apt install flatpak   # Installs flatpak to your system
flatpak remote-add --if-not-exists flathub https://dl.flathub.org/repo/flathub.flatpakrepo   # Adds Flathub, the biggest store for flatpaks

Once it’s there:

flatpak install librewolf

Someone using Linux for years might know where stuff on system is placed and not fear not knowing what a command do and how to undo it. But if you don’t know what is happening, better to stick to distribution provided sources. Otherwise the equivalent would be like typing some commands in Windows to change registry keys :). I think Librewolf should recommend Flatpak by default instead.

Sorry if this is too much info, just tried to explain things a little more than usual.