I have the following problem: DO retains as tabs, and obviously accesses at each startup, all visited folders (including FTP). This slows up startup times and is a function I would rather not have active at this time.
Opus doesn't open any extra tabs by default so they must be opening due to some specific action or configuration change.
What are you doing which opens the tabs? If they open when you double-click on a folder outside of Opus, turn off Preferences / Launching Opus / Explorer Replacement / Open external folders in a new tab (although you'll then get windows opening instead of tabs; either way you'd have to close either the windows or the tabs when you no longer want them open).
You may also want to change some of the other options in the Launching Opus part of Preferences. e.g. Turning off Default Lister / Update Default Lister automatically when closing a Lister or setting Startup / Don't open any Listers. It depends where the tabs are coming from, though.
If you're getting tabs opening from actions you perform within Opus itself then your config must've been modified, e.g. to make double-clicking on folders within Opus open them in new tabs. (Or your Alt key is stuck down.)