Moving from Xcode 11 beta 4 to beta 5, you may get a warning when you have borders in your code:

    Text("In publishing and graphic design, lorem ipsum is a placeholder text.")
        .border(Color.green, width: 2, cornerRadius: 16)

The warning is:

    'border(_:width:cornerRadius:)' is deprecated: Use a RoundedRectangle shape.

Here's a copy-paste solution to the above warning:

    Text("In publishing and graphic design, lorem ipsum is a placeholder text.")
        .background(RoundedRectangle(cornerRadius: 16).strokeBorder(Color.green, lineWidth: 2))

In a condensed form, I had the following SwiftUI code:

    struct Input {
        var someString = "OH HAI"
    }

    struct ContentView : View {
        @State private var input = Input()
        
        var body: some View {
            TextField("Enter string", text: $input.someString)
                .textFieldStyle(.roundedBorder)
                .padding()
        }
    }

Then I changed someString to a computed property:

    struct Input {
        var someString: String {
            return "OH HAI"
        }
    }

This results in the following error: "Generic parameter 'Subject' could not be inferred". It took me ten minutes to realize what was the problem: I forgot to add a setter.

To print the current date in ISO 8601 format on macOS:

  $ date +%Y-%m-%dT%H:%M:%S
  2019-04-23T14:44:46

To print it with the timezone information, append %z.

  $ date +%Y-%m-%dT%H:%M:%S%z
  2019-04-23T14:44:42+0200

Note that in the last case, you're missing the colon in the timezone bit. To correct this, we'll need to do some bash wizardry:

  $ zone=$(date +%z);datetime=$(date +%Y-%m-%dT%H:%M:%S); echo $datetime${zone:0:3}:${zone:3:2}
  2019-04-23T14:44:42+02:00

My current list of Firefox extensions on my Mac is as follows:

• Dark Reader
• Disable WebRTC
• I don't care about cookies
• uBlock Origin

Plus I use the Shadowfox to style Firefox a bit more to the liking of macOS Mojave's Dark theme.

And finally to fit things in perfectly, put this bit in your userChrome.css to move the tabbar close button to the left:

  @namespace url(http://www.mozilla.org/keymaster/gatekeeper/there.is.only.xul);
  @-moz-document url(chrome://browser/content/browser.xul){

	  #TabsToolbar #tabbrowser-tabs .tabbrowser-tab:not([pinned]) .tab-close-button {
		-moz-box-ordinal-group: 0 !important;
		-moz-margin-start:-4px !important;
		-moz-margin-end:4px !important;
	  }
  }

I found out about two extremely useful Xcode shortcut keys: Ctrl+Cmd+[ or ] to cycle through the active target, and Ctrl+Option+Cmd+[ or ] to cycle through devices.