I deactivated all of my plugins and tried a post. It went blank as before.
So then I shut down FF, then restarted it, and tried another post. That seemed to fix the problem.
Then I restarted all of my plugins and tried a post. It still worked.
Then I restarted FF. And it still worked.
That doesn’t make a lot of sense to me.
it worked for a while. but after a few minutes of random browsing, my next post failed (all plugins enabled, etc). i’ll have to try it with all of them disabled for a longer time, to get a true test, i think.