I have completed tests on 3 seperate boxes (barebone not VM) installing with DVD media with an external DVD player (ASUS 90-DQ0435-UA221KZ)
- Dell Optiplex 7060 i7-8700, 32 GB RAM booting uefi+secure boot
- Dell Optiplex 7040 i7-6700, 32 GB RAM booting uefi
- Dell Latitude 7280 i5-7300U,16Gb RAM booting uefi+secure boot
Booted into live desktop and waited for the dvd to complete (stop spinning) - then checked Internet menu and selected Firefox which ran after a few moments - proceeded from there to install the OS - all went well and the installs took 3 to 5 minutes depending on the machines.
Interestingly I found that it took ca 45 minutes on all boxes to complete loading the Live desktop. When installing from USB media on these machines the boot time is ca 2 minutes.
So the result is I could not reproduce the reported errors re:Firefox on these machines.