The name of this add-on says it all. Once installed simply double-click on the tab to reload. Reload Tab On Double-Click is about 2 KB and works with Firefox 1.0-3.0.X (but can be forced to work with Firefox 3.1/3.5 via Nightly Tester Tools)
Source: Mozilla Links
If you have tab mix plus installed this feature can be created in the tmp options.
Ah okay, I knew that behaviour sounded familiar.