Tab Mix Plus
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Full user review
"Really useful.... although still a little confusing ;-)"
Allows the control over Tab behaviour Firefox doesn't offer (anymore?). Covers about every possible options one can think of.
The interface and learning curve are still not the most user-friendly. While I waited for the FF3 version, I forgot how most things were working and had to learn them all over again when it was finally issued. Still doesn't spoil the Pros!