You can check in the Rules tab in the right panel in the Inspector what font-family is used for selected text. You can check in the Font tab in the right panel in the Inspector what font is actually used because Firefox might be using a different font than specified by the website.

• https://developer.mozilla.org/en-US/Tools/Page_Inspector/How_to/Edit_fonts

Thanks a bunch! I did not know about the Font tab in the Inspector. That allowed me to identify that weird font as Microsoft YaHei, a Chinese language font; no wonder it looked so off.

The next thing that intrigued me is that YaHei was set as the Sans-serif font for simplified Chinese, not Japanese; a bit of testing confirmed it was the parameter used to render the text I was looking at.

Turns out I needed to add Japanese to the list of "Choose your preferred language for displaying pages", which initially only contained English. the parameters for the Japanese language were then properly reflected. I guess Firefox defaulted to simplified Chinese to render Japanese text then.

Mystery solved, thank you very much!

Hey guys I was having the same problem as MattP and found this thread. I still can't get to fix mine though.

So I used the Chosen Solution's method to find out which font is messing up. Then I used MattP's method of adding Korean(the font im having trouble with) to the list of "Choose your preferred language for displaying pages", but nothing would change.

What should I do now? How can I change the font or the parameter used for rendering?

PS - even the texts on the Tabs are displayed with the same ugly font.