Actually since you use Firefox normally that's perfect for what you want b/c you can install chrome, apply the dark mode flag, bookmark QBO in Chrome, drag the bookmark to your desktop, & then use Firefox as your web browser but when you need QBO click on the Chrome bookmark saved to your desktop (if the bookmark wants to open in Firefox then you may have to first open Chrome & then click your bookmark). Alternatively you could install Brave (a privacy focused Chrome derivative) & do the same thing or you could use multiple chrome profiles (accessed via settings), the flags should only apply to one Chrome profile at a time.