I now have users on my site saying the same thing now...
I know it isn't my site as it was doing it to me here on XF
I tried on another PC and that was ok
Users are having the same problem
I fixed it by reinstalling Firefox which is ok for me but a bit silly having to tell every user on my site that if you are using Firefox and are experiencing this issue you have to reinstall Firefox to use my site.
Not sure what to do now and what is really causing the issue