How can I make Dolphin open bookmarks in the current tab?
In the current version of Dolphin:
Menu Button > Settings (ship's wheel) > Customize > Open links in > By default

Default will open in the page is set up to open the link into (normally the same page), other choices are Open in background tab (current tab remains the active tab), and Open in new tab.   

Older versions of Dolphin had option under Labs instead of under Customize.

Optional:

I find it best to stick with the default open in the same tab, but will often use a bookmarklet to force tabs to open in a new tab when I want to make sure I don't fail to use the context menu to open a link in a new tab, for which I assign a gesture written as "nt" in Dolphin, or a keyword shortcut of "nt:" in Firefox (where : is not hard to type on a keyboard).

Name: &nbsp; &nbsp; NT:  New Tab
Address:  javascript:(function(){var%20a=document.getElementsByTagName('a');document.title='||'+document.title+'||';for(var%20i=0,j=a.length;i&lt;j;i++){a[i].setAttribute('target','_blank');}})();

When creating the bookmarklet in Dolphin, create by replacing content of a newly created bookmarklet with the above content, typing a space instead of  %20 and  and suggest changing the folder to one you have created as "bookmarklets" within your bookmarks.

see my page on Dolphin Gestures