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