Font weights appear indistinguishable [duplicate]

Question

Code Review is the only website on which font weights behave weirdly - for me (using Linux Mint 18.2 and Firefox 55):

1. Some bold text: hello world.
   ... but it doesn't seem bold! It's the same weight as the rest of the text.

2. Headings:

   Level 1 heading some code

   Level 2 heading some code

   Level 3 heading some code

   the heading text has regular weight (unlike the norm on SX website), while the source code snippets in headings - is in bold.

I'm not the only one seeing this, right?

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Screenshot of the above:

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