This was more of an accident than an experiment.

Firefox-3.4.5

So, I had just installed Firefox 3.5.4 and was so exhausted, I dozed off a bit with my fingers on the ctrl+t. Now, when I woke up after a few seconds, I saw a thousand tabs lined up in the window.
Next, I pressed ctrl+w and waited. As the tabs were closing down, I saw a few flashes. Then as I reached the last tab, I saw no page in it! The last tab, which was my first tab with Google opened in it, was not there anymore. To the best of my knowledge, Firefox opens tabs to the extreme right.

Not believing what happened, I decided to try it once more. Started a new instance of Firefox, opened Google in the first tab and opened a number of tabs after that by holding down ctrl+t. Now, after around 20 seconds, I started closing them down with ctrl+w. This time too, amazingly I saw a page flash by among the tabs I was closing. Aren’t all tabs supposed to be empty?

And when I reached the last tab, I could see my Google page open in it. But this time though, it closed down automatically after sometime and took down the Firefox instance with it. That is obvious because I did not tweak Firefox to stay even if the last tab was closed but the rest!

Is this some bug? Might be some limit with the data type they are using to keep track of the tabs.