Hi, there appears to be an issue that only affects users who have turned off Cleartype Font rendering within their Windows Settings. So a workaround for the moment would either be to turn Cleartype back on or disabling hardware acceleration in Firefox: https://support.mozilla.org/en-US/kb/performance-settings

Please let us know if this solved your issue or if need further assistance.

That's strange. If you use the Page Inspector to check some text on the page, can you see what actual font it is using? To open the Inspector, right-click some text and choose Inspect Element. Firefox usually will open the Inspector in the lower part of the tab, show an HTML tree, and select the element you right-clicked. On the right side, there will be something displayed, usually Rules. View fonts on that side to see what the site is using. If Fonts is not visible after Rules, Computed, Layout, etc., look for a little triangle at the end of that row of headings.

If you go to this folder in Windows Explorer (not in Firefox):

C:\Windows\Fonts

Does Arial look normal? It's a family, so if you double-click it, Windows should display all the members of the family.