I think you've answered your own question now (that the two screenshots are showing different fonts/weights)! This is a FIREFOX BUG, I reported it to them a long time ago, and after at least two years it's way past time they got their act together and fixed it. The web page tells the browser what fonts to use and the browser should use them - if Opera can find and use them on my system, FF should too. Given that Opera gets it right, I'd say this ISN'T an issue with system fonts, especially since I have a complete set of Microsoft TTF fonts installed on my Linux box. I've asked before, and I'll ask again here and now: How is it that Firefox can get it right in Windows, and Opera can get it right under Linux, but Firefox can't get it right under Linux? And the thing that drives me completely insane is that, if you install Opera under Linux and compare its font rendering with the same page in Firefox under Windows, the pages look virtually identical. Not ideal, but better than the Firefox defaults.

I've been trying to fix this problem for two years now, and my only solution has been to force Firefox to use my selected fonts.

Firefox 45.0 blur bar