My userChrome.css doesn’t seem to be doing what it did previously, and the text in my URL-bar is far too small. Here’s what fixed it in my userChrome.css previously:

#urlbar {-moz-appearance: none !important;
    font-family: Tahoma Bold !important;
    color: Black !important;
    font-size: 16pt !important; }

but it doesn’t seem to be doing anything now; change the font-size and restarting Firefox doesn’t seem to make any difference. Why is it behaving differently now?

Answer: font-family

It turns out the main source of the problem was actually the font-family (which was Tahoma Bold), and apparently locks you to a certain size. Removing the font family line fixed the problem. I discovered this by using the browser toolbox: =Menu -> Tools -> Browser Tools -> Browser Toolbox.

Removing the * {font-size: } line probably also helped, but didn’t solve the problem until I removed that font-family.

  #urlbar {
    -moz-appearance: none !important;
    color: Black !important;
    font-size: 1.5rem !important;
}