You can right-click and select "Inspect Element" to open the Inspector (Firefox/Tools > Web Developer) with this element selected.

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

You can check the font used for selected text in the Font tab in the right pane of the Inspector.

cor-el said

You can right-click and select "Inspect Element" to open the Inspector (Firefox/Tools > Web Developer) with this element selected.

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

You can check the font used for selected text in the Font tab in the right pane of the Inspector.

It appears to be Helvetica Neue. Anyway, other fonts like Mozilla's very own FF Fira Mono look much thinner and terribly rendered with FF40 as well. I can reproduce this with the respective portable versions of FF39 and FF40. The upgrade causes a significant change in how fonts are rendered.

For some reason, Direct 2D was deactivated. I've seen bug reports linking this to Nvidia drivers, but I don't have that kind of hardware. So, I opened about:config and set

gfx.font_rendering.directwrite.enabled to false and mozilla.widget.render-mode to 6.

If the last entry is missing, create a new integer.

Everything's fine now.

Oh, and don't forget to restart the browser! :)

Sorry, I made a mistake. This is the correct solution:

Set gfx.font_rendering.directwrite.enabled to true and mozilla.widget.render-mode to 6.

I had this problem with Windows 8.1. Turns out the only Helvetica Neue font I had installed was the Bold type. Apparently Firefox, unlike Chrome, chooses this variant even if the website didn't ask for it.

I just deleted the font from my system and that fixed the problem.