When there is an url in MARC field, and you want to browse to it, there are now (as of version 246), two ways to do it.
In either case, the field must be selected in order for the desired browsing action to take place.
About <F12>
The <F12> key was formerly used for 'Save current record as a Template'; that action is still available as a menu option under the 'Edit' menu. In the current version, the <F12> function key has been defined so that pressing it while editing will launch a web browser on the first url found in the field being edited.
If there is no url in a field and <F12> is pushed, nothing will happen.
About double-click
Double-clicking on an url is a common way of telling an application to browse to that url. This behavior is controlled by an option, enabled by default, on the Record Display page of the Options.
When double-clicking, the cursor must already be positioned in the field containing the url. For example, in the following snippet, the url is the 856 field, but the 700 field is selected:
To browse to the link in the 856, one will have to first click on the 856 to select it, then double-click.1)
If there is no url in a field being double-clicked, the program will instead select all of the text in the field.
When double-click browsing is disabled in the options, double-click will always select all of the text in the field.