This started happening sometime after I opened the first tab and before I opened the second tab in the before/after images in the OP. I did not see this happen before then.
Specifically, I opened the New section in a new tab, which looked like the screenshot labeled Before.
I then opened a few threads/links from there in a new tab, each of which had the font weight issue.
After this, I opened the New section in another tab specifically to check whether this is really happening.
In that time, I did not intentionally or knowingly perform any action related to theming, and only opened links.
With this change came a few layout changes, such as the DevForum logo at the top left moving slightly to the right and a footer being added to all pages. I assumed these were intentional and the font weight change was unintentional.
I was on an old version of Firefox on xfce on Arch Linux. Between creating this thread and posting this reply, I have upgraded my browser and system and the issue persists. (I will have to verify whether this still happens when logged out of the devforum)
When I open devforum.roblox.com on a mobile device (i.e. not this browser, not this device), the styling is normal.
I would like to highlight that the font weight changes as soon as the page loads and is normal during loading.