Where to find copy of Mac OS Mojava?
Where to find copy of Mac OS Mojava installer? iTunes states that copy is not valid.
Did you try the Mojave link in this Apple article:
How to get old versions of macOS - Apple Support
I tried it early today and it seem to be working.
The "not vaild" comment could mean you have an older Mojave installer already in your Applications folder. If so, remove it and try again.
Also, any downloads of Mac system software need Safari. There are many reports over the last six months of FireFox not working for this.