‘Save & Activate’ in the Customizer tries to save then stops

I’m using Firefox for this.
I’m trying to copy my settings from my other two blogs, hosted on my website. The two I have both use the Twenty Ten theme (I like the simplicity and it works well for my needs). But this also affects other themes (even the latest Twenty Seventeen theme).
The issue happens in the Customizer (and only seems to be this. My other two blogs work just fine(tested them with what I’m doing and no issues)). Hitting the Save & Replace button attempts to save for like a split second, but it doesn’t gray out with the button changing to ‘Saved’ (unlike my other two blogs).
I tried other themes. I tried disabling my only plugin (Disable Comments). I tried refreshing Firefox’s cache. I tried restarting Firefox. Nothing’s working.

I managed to activate the browser’s Console, and noticed this error coming up everytime I click the button;
TypeError: this._recipeManager is null[Learn More] LoginManagerParent.jsm:104:9
Since I’m no coder, attempting to look this up gives me a bunch of alien gibberish.

Another thing up with this is when setting up the Widgets. Typing in the title of the widget I want, does a loading thing that lasts forever. It’s like it’s trying to do it’s automatically update the page thing, but it cannot (which seems to be the same for the Save & Activate not working issue). Not seeing an error in the console when this happens.

Please assist. This is a fresh install.

The page I need help with: [log in to see the link]