Do you have the default theme enabled in Firefox?

You can check the current theme via the "Themes" button at the bottom of the Customize palette window.

• Bug 1466335 - Automatically enable the dark theme in macOS 10.14 dark mode

(please do not comment in bug reports
https://bugzilla.mozilla.org/page.cgi?id=etiquette.html)

Usually I have the Light Theme enabled. Why? Does it only work if you have Default Theme enabled first?

[reads the bug report]

Oh wow. So if you pick the Light or Dark Theme, it stays like that, but if you pick the Default Theme, it switches to appear like Dark Theme when macOS Dark Mode is enabled. Well, that's definitely something that should've been mentioned in the changelog. And also seems like a poor way to design this, since it means that the user is forced to use Default Theme all the time in order to take advantage of this behaviour. I believe a far better solution to the problems they were discussing would have been letting the user decide if they want to use this behaviour at all, and if they decide to use it, have it work regardless of what theme they use in macOS Light Mode. Or even let them decide both themes.

Oh well, at least now it works. Thank you for your help!

SavannahJ said

since I'm on Firefox 62.0.2,

Since you are using Mac OSX 10.14 Mojave you should use Firefox 62.0.3 as it has a fix hangs when various dialog windows are activated.

https://www.mozilla.org/firefox/62.0.3/releasenotes/