If you are using outlook.live.com to read your @hotmail email sorry to tell you that you are out of luck. I lost the spell check many months ago as did many others when Microsoft made changes to all the various things they now call outlook. For me the spell check doesn't work using Chrome browser either. Only have those 2 on my computer. I did find a way to get the spell check to work, but only after the entire email has been typed. If you add any word or even punctuation it stops working again. To spell check after the email is already typed click on the options tab at the top of the screen, click on the blue customize your editor on the left on the next row down, click editor settings, ensure that under the corrections heading that spelling and grammar are turned on, click OK and you should see all the miss-spelt words red underlined. This is incredibly frustrating but it is a Microsoft screw-up not a Firefox one. I sent them feedback more than once, but they never replied. There is also no help whatsoever for us @hotmail users in their forum or in their chat option either.