Hi. Whenever I try to use the custom preview option on any font, it results in a blank page. Trying to sort fonts or filtering them (only as/only fonts with) also comes up with a blank page. I am using Firefox on Windows 10. Any solutions would be appreciated.

any extension installed on Firefox? Try disabling all extensions to see if it works as expected.
(i'm running FF on W10 just like you, i have no problem)

That was it! It was the add-on "ClearURLs". Thanks!