Type about:preferences#content<enter> in the address bar. Across from fonts and colors, press the Advanced button. On the bottom, turn on Allow Web Sites To Choose Their Own.

You can check for corrupted and duplicate fonts and other font issues: http://www.thexlab.com/faqs/multipleappsquit.html - Font Book 2.0 Help: Checking for damaged fonts http://www.creativetechs.com/iq/garbled_fonts_troubleshooting_guide.html

The sidebars use:

font: 13px/1.25 "Helvetica Neue",Helvetica,Arial,sans-serif;

Or in other words:

font-size: 13px; line-height: 1.25; 
font-family: "Helvetica Neue",Helvetica,Arial,sans-serif;

Did you install Helvetica Neue? I believe it is a heavier typeface than traditional Helvetica/Arial, but it should not be oblique (slanted).

I'm not sure about Windows 10, but on Windows 7 if you open this folder you can click through to see which of the various faces are available for your different fonts.

C:\Windows\Fonts

For example, if you double-click Arial, you can see numerous different styles and weights.

What do you find for Helvetica Neue? Did you get a regular weight and bold weight that are not oblique?

You can right-click and select "Inspect Element" to open the Inspector (Firefox/Tools > Web Developer) with this element selected. You can check the font used for selected text in the Font tab in the right pane of the Inspector.

• https://developer.mozilla.org/Tools/Page_Inspector