Hi, the sites you mentioned and Disqus default to Helvetica Neue font if it is found in your system. Perhaps it is then a problem with the particular font. Helvetica Neue is not included in Windows by default. You can

1. uninstall the font in your system. EDIT. Uninstall "Helvetica" as well if uninstalling Helvetica Neue isn't good enough. Many sites, some of the ones you mentioned too, default to Helvetica if Helvetica Neue is unavailable. Helvetica isn't included in Windows by default either.
2. or create a rule for Firefox to render Helvetica Neue as some other font, for example Arial.

To uninstall the font, read this.

To keep the font but use Arial instead in Firefox, create a userContent.css file as instructed here.

Copy this content to the file (taken from here):

@font-face { font-family: 'helvetica neue'; src: local('Arial'); }
@font-face { font-family: 'helvetica neue'; font-weight:bold; src: local('Arial'); }
@font-face { font-family: 'helvetica'; src: local('Arial'); }
@font-face { font-family: 'helvetica'; font-weight:bold; src: local('Arial'); }
@font-face { font-family: 'HelveticaNeue-Light'; src: local('Arial'); }
@font-face { font-family: 'Helvetica Neue Light'; src: local('Arial'); }

Save and restart Firefox.

(If you have further questions, Finnish is ok, if you prefer.)

Thanks, replacing the font with Arial worked. I tried uninstalling and re-installing Helvetica, but that wasn't doing anything.

Ok. I suppose uninstalling needs to be followed by clearing the font cache and rebooting the system. However, the end result for Firefox would be about the same, with the difference that with the replacement trick, Arial is always used when Helvetica Neue or Helvetica is a website's default, whereas after uninstalling Helvetica Neue and Helvetica, Arial might not always be the first alternative on such websites.