Hmm, it certainly looks fine.

When this happens, do you recall whether you had typed the address, pasted the address, or had Firefox auto-fill the field from the form history drop-down?

I type the emails by hand. In other websites sometimes erasing and starting again works, but for this particular situation above that trick didn't seem to work. I just had the same thing occur on Overleaf, but when I erased and retyped it worked.

I think I figured out what is happening. I think I am putting spaces at the end of the email (a habit I must have had for years), but now the html submit email form field doesn't seem to accept spaces afterwards anymore and marks them invalid.

Hmm, yes, that could explain it. But possibly the website doesn't care about Firefox's opinion about the address (if it's Firefox's opinion) and would let you submit the form anyway. You might check that the next time -- it would be nice if browsers were consistent on this point.

Usually the websites are not letting me submit, claiming the email is invalid. What I find perplexing about this is that it just started like a month or two ago and was not a problem before that.