Some elements are not centrally aligned:

[Aterfax]

See:

[boardfish]

That's odd. The HTML on both sections is identical, too. Can you replicate this after clearing cache etc.?

[Aterfax]

Shift refresh still has the issue - current version of FF 62.0 on Windows 10 x64.

[Aterfax]

This element is at fault?

<div class="text-center text-light">
<h2>Our committee loves games!</h2>
<p class="lead">Our committee loves games just as much as the rest of the society! We share many roles - arranging events, setting them up, providing food and consoles, and more. Head <a href="committee.html">here</a> to see what we're all about.</p>
<a class="btn btn-primary btn-lg w-100 mb-3" href="/committee.html">
<span>
<span>
Learn more
</span>
</span>
</a>
</div>

[boardfish]

I'll investigate further.

[Aterfax]

Not entirely sure myself but Firefox might be interpreting a sequence of CSS that is nested down each element differently than expected?

[boardfish]

Can't reproduce this on Firefox 62 (Arch Linux). Closing until it can be accurately reproduced on a more mainstream OS.

[Aterfax]

This should not be closed, issue unresolved.

Inability to currently reproduce does not warrant closure.

[robjtede]

I have reproduced. Fixed by adding the mx-auto class.

[boardfish]

I was fairly sure it had this already, but sure.