Using Firefox Nightly for Android because it is the only Firefox version with support for custom SSL certificate authorizations now.
Go to the InvoiceNinja site you’ve hosted and login, then click your little context button/hamburger 
button, and there will be an ‘Install’ option to install full screen web app shortcut launching the app through Firefox Nightly.
Very lightweight and functional ‘isolated’ mobile app experience. It runs as a separate app from primary Firefox, with no other tabs, this app can be pinned if you want it to act as “till device” or etc. Same can be done with these forums. Enjoy.