I've been reading my Kindle books through Firefox just fine for several weeks now.
Yesterday I closed the tab on which I was reading, came back later, opened a tab, went to the https://read.amazon.com/about page to sign in--though I just click on "Sign in" since it remembers my info, which then goes to the book I'm currently reading--and where I normally see my book I now get nothing but a *completely blank* display.
I've restarted Firefox, and rebooted my PC. I can do a "View Source" on the blank page, which verifies that there is code there--I'm just not seeing the book.
Going through Kindle Troubleshooting it recommended uninstalling the Reader App and reinstalling. Unfortunately the means to do that resides on the blank page, so it's inaccessible. I've looked through FF extensions and Add-ons and haven't spotted the app, so I don't know where else to look.
I would try signing off and back on, but again, the "log off" link appears to reside only on the blank page.