I was just watching episode 180 of DL.TV and they featured a hack that would allow you to disable the close tab button for individuals that accidentally close their Firefox browser tabs while they are clicking back and fourth between them.
If you have watched the episode, there is a much simpler way of recovering an accidentally closed tab than doing the whole "about:config" method. If you haven't watched the episode, the method involved typing "about:config" into the address bar and changing the numerical value for "browser.tabs.closeButtons" from 0 to 1 or 2 (2 disables the button all together).
Unfortunately, this hack doesn't help you recover already closed tabs, it just prevents you from closing tabs in the future. If for some reason you ever close a tab by mistake and you do not remember the URL of the site you were on (this happens to me all the time), you can easily recover the lost tab and session by pressing the following key combination:
CTRL+SHIFT+T
Not only will this bring the last closed tab back to your screen, it will place the tab in the same position it was in before you accidentally closed it and you can continue to press CTRL+SHIFT+T to bring up all of the tabs that you closed since you opened the browser. It's like the "CTRL+Z" command of tabbed browsing and without it, I would be lost.
It should also be noted that you could theoretically use this feature to spy on someones browsing activity if they always leave their browser open, but that would be naughty and everyone would shake their finger at you in disappointment so don't do it! ;)