You've undoubtedly seen an image like this millions of times.  I certainly have.  To open a new tab, you simply click "+".  And I pressed "X".  Just a few minutes ago, in fact.  Minutes that have been spent staring at the screen, wondering where everything just went and why I did what I did.  And there was anger.  Oh yes, much anger.  And somewhere off in the distance, a plaintive plea: "Mister, can I have my ball back?"

So, yeah.  F1 cars.  They're neato-keen!

1 I did that just the other day.  I'm so sorry.  That sucks.

2 I did this with the research for a twenty page plus research paper once. Had a heart attack until I remembered control+shift+t on Chrome restores tabs. 

3 Been there, done that, had QWERTY imprinted in my forehead afterwards..

4 GAAAAAAAH.

Been there, cursed at that. *nod nod*

5 Is there an extension that will save the contents of tabs in case this happens?

6 The version of Firefox I'm using will reopen a closed tab, but I think it launches a fresh copy, not actually pull a recent cached version up.  I'll have to check.

7 No need, it's a fresh copy.  Otherwise, I wouldn't have had to post this post in the first place.

8 Allow me to repeat my recommendation for "Text Area Cache" as a Firefox extension. I have it set to keep the last 100 text areas, which they think is a bit much, but works fine. The only site that gives me trouble with that is Facebook, because it re-uses page elements, I guess. I don't think it records the "More" area, but the main post works. And of course, comments.

9 Ugh.  I've done that too many times in Chrome by hitting backspace when I wasn't in the text box.

10 "Good" news, Google hears your pain, Pixy, and broke twenty years of back-compat by disabling "backspace -> history.back()".
Now, if you hit Backspace, Google says "we know what you wanted to do, but we're not going to do it."

11 That's something I make a point of disabling in Firefox. No unintended navigation allowed. Have to turn it off in the mouse software too.

12 I'm sorry to hear that happened to you.

I had something similar happen to me when I'd just started anime blogging - undershot the minimize button and hit the tab close button. Couldn't even look at my blog for days without wanting to throw something. I got paranoid about it happening again and started writing everything out in MS Word first (with autosaving every 5 minutes turned on).

Having two completely different functions like that together is really poor design. I've stuck the new tab button all the way to the left because at some point that was the original default and I didn't like them changing things on me and now I see a really good reason why it belongs to the left of the tabs.