If your problem is the way I think it is, you might try the following:
Open a terminal in the AppVM you want to use the firefox. Set the following two variable and open firefox by typing into the terminal:
export GDK_SCALE=2
export GDK_DPI_SCALE=0.5
firefox
Does this look much better? These settings apply scaling for all GTK-based applications (so also the “files” application, the pdf viewer etc.). I found this to be necessary, although it is not included in the documentation.
If you like what you see, you can make this permanent for all AppVMs by creating a file into your template vm. For this you need to create a file /etc/profile.d/dpi.sh in your template vm (e.g. fedora-34) with the following content:
#!/bin/bash
export GDK_SCALE=2
export GDK_DPI_SCALE=0.5
This makes these setting permanent for all AppVM and all their applications.
If this is not helping you or it is not what you are looking for, please let us know what exactly you tried so far and what templates you are using and may even provide a screenshot or similar.