Your System Details shows;

Windows 10 Firefox 49.0.2

But your User Agent shows;

User Agent: Mozilla/5.0 (Macintosh; Intel Mac OS X 10_11_6) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/54.0.2840.71 Safari/537.36

What is your computer system and Firefox version?

Many site issues can be caused by corrupt cookies or cache.

• Clear the Cache and

• Remove Cookies
  Warning ! ! This will log you out of sites you're logged in to.

Type about:preferences<Enter> in the address bar.

• Cookies; Select Privacy. Under History, select Firefox will Use Custom Settings. Press the button on the right side called Show Cookies. Use the search bar to look for the site. Note; There may be more than one entry. Remove All of them.
• Cache; Select Advanced > Network. Across from Cached Web Content, Press Clear Now.

If there is still a problem, Start Firefox in Safe Mode {web link} A small dialog should appear. Click Start In Safe Mode (not Refresh). While you are in safe mode;

Type about:preferences#advanced<Enter> in the address bar.

Under Advanced, Select General. Look for and turn off Use Hardware Acceleration.

Poke around safe websites. Are there any problems?

Then restart.

Thanks for the reply.

My OS is Windows 10 VM set up in a MAC machine, so this is the reason why it shows Mozilla/5.0 (Macintosh; Intel Mac OS X 10_11_6) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/54.0.2840.71 Safari/537.36 as User Agent.

I've actually tried all these already, but any of those fixed the issue.

Thanks

iliWebDev said

but any of those fixed the issue.

I think you meant; 'None have fixed the issue,' yes?

Start your Computer in safe mode with networking. Then start Firefox. Try Safe websites. Is the problem still there?

Starting The Computer In Safe Mode;
Free Online Encyclopedia

Type about:support in the address bar and press Enter. Under the main banner, press the button; Copy Text To Clipboard.. Then in the reply box at the bottom of this page, do a right click in the box and select Paste. This will show us your system details. No Personal Information Is Collected.

Yes, I meant that none of these fixed the issue. I've just tried the suggestions above, but I'm still having the issue.

It looks to me that is something with the focus on the page. As long as I shift the focus to somewhere else and then go back to the input fields again, I'm able to type anyhing as normal.

Thanks

I have this issue only in the three latest versions of Firefox (49, 50 and 51). It works as expected in any older version.