I just tried using web mail here using Firefox 57.0 (64-bit) Quantum with no issue.
Make sure your add-ins are up to date, especially Flash.
I just tried both HughesNet's online email and Outlook online and both worked without issue.
I had the same problem. I contacted HN CS and an agent worked with me patiently with over a hour or so trying to rectify the problem. He then connected me to Advanced Support and John fixed the the problem. Click on the Preferences tab in HN Mail, then click on Composing, then Composing Messages, make sure under composing messages the "as Text" is checked. Then click save. Evidently there was a glitch in the Firefox update. Thank you HN!!!!
To the person who claimed setting to "text" fixed it: Evidently it did something, but not what you think it did. I never had mine set for text and it works just fine. So, not a glitch in Firefox.
Tara in customer support answered my email quickly. Solution for me was to go to Preferences, General, and toggle between Standard version and Advanced (ajax). Restart email program. I had stumbled upon the change to text fix myself--however it was not a real fix. Thanks Tara!
Only thing that makes sense since I hadn't used it prior to now (I use a POP connection in Outlook), so nothing prior to get scrambled. That's my guess (and I'm sticking to it). 🤷🏻♂️