Have you tried:
- Send to Kindle app (for macOS or Windows)
- use iOS Sharing, choosing Send To Kindle on your iPad or other iOS device (with Kindle app installed)
Neither of these depends on email white-listing for delivery, and the file size limit is less restrictive (50MB last time I checked).
It has been awhile since I’ve used email to get stuff to Kindle so if that is broken I would not know.
As for Send To Kindle browser plugins for Firefox - thought amazon had one (or maybe it is just for Chrome?). That might be better, again, as Amazon’s plugin knows how to get stuff to one of your devices without you having to give it an email address.