I can make very little changes to allow syncing the tabs using firefox account, I will simply add it to next version. However, I am more interested in your use case. In my understanding, this feature helps content switch during working/browsing. It is like temporary bookmarks, but easier to use than bookmarks. If you want to sync between different browsers, that means they are not temporary, which I think bookmark is probably a better solution and it supports sync over firefox account natively.