Per above, I had Dracula working on a non-snap install of Firefox, so that the close / minimise / maximise icons were consistent with the theme.

I stupidly tried to select a folder colour using System Settings - Appearance after having no luck using Folder-Color with Nautilus. This changed my theme back to the system default, and changed Firefox's theme at the time.
I have uninstalled and reinstalled Firefox, ensured it is not a Snap installation, installed using the binary from their website, and have had no joy getting the app to recognise Dracula as the GTK Theme.
Any hints or recommendations would be very much welcome!