Totally fair question. I've actually been a longtime Gecko/Firefox user myself, so this one stings a bit.
The short answer: Firefox doesn't support the File System Access API (https://caniuse.com/?search=File+System+Access+API).
We made a deliberate decision to go client-first. Video editing happens entirely in your browser without us uploading your entire footage on our end. No bandwidth costs for you, no storing your raw video on our servers. The File System Access API is what makes that possible, and unfortunately Firefox just doesn't have it yet.
It's not a forever thing though. For cloud-based projects where files live on our end anyway, Firefox support is very much on the roadmap. But for the local-first editing flow, our hands are a bit tied until Mozilla ships it.
Hope that makes sense, and fingers crossed Firefox adds support soon!