I have a similar system. 1.80 gigahertz AMD Sempron, 960 MB RAM, running XP-SP3. I use "open in a new tab" quite a bit but I don't remember anytime that it caused the current tab to close.
In EDIT - PREFERENCES - BROWSING - TABBED BROWSING - GENERAL - OPEN NEW TABS, I have "Next to the last tab" chosen. What do you have there?
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Bugs