sued, when you set the Double-click option to Open the Location in web browser (on the General tab of 1Password preferences), 1Password displays the three supported browsers, as shown below. You select a browser in that list. (Note that Safari is not a supported browser.)
If you click the OK button to save your preferences, double-clicking on a listed Login will launch or open the selected browser and point it to the URL listed in the Location field of the double-clicked Login. From there, you can press Ctrl+\ to fill (and optionally submit) the form at that URL.
(I've just confirmed this behavior in the latest build of 1Password for Windows.)