Best solution for that:
Install chrome
Apply the dark mode flag
Bookmark QBO in Chrome
Drag the bookmark to your desktop
Use Firefox as your web browser
For QBO click on the Chrome bookmark saved to your desktop
If it wants to open in Firefox, you may have to open Chrome first & click your bookmark in Chrome Alternatively, install Brave (a privacy Chrome derivative) & do the same
Alternatively, using multiple chrome profiles (accessed via settings), may work
