This has to be one of those rare features that I can say in all my years of using Firefox, I’ve never used or even found a need to use. Talking about the ‘Close Other Tabs’ option in the context menu when right-clicking on a tab. As the name suggests, it close all the other (except App Tabs) tabs while keeping the current one open. Chrome has this feature as well along with an option to close tabs to the right. Apparently some folks rather right-click on a tab and select ‘Close Tab’ instead of clicking the X and these same folks might be a little off on their aim, thus clicking on the Close Other Tabs option.
Back in January 2005, Bug 277079 was created asking that users be warned if they select the Close Other Tabs option. The bug was marked as RESOLVED/WONTFIX the next day saying there is already a warning and that was the end of that. However, this past January (7-years later) the bug is suddenly reopened and by July a patch was created which landed on the Firefox 16 Beta. This patch will display the warning when Close Other Tabs is select even if the user has already turned off the Close Multiple Tabs Warning (Tools > Options > Tabs > Warn Me When Closing Multiple Tabs) or via about:config setting both browser.tabs.warnOnClose and browser.warnOnClose to false.
An interesting side effect to this patch is it now conflicts with the Close Tabs To The Right add-on. The new ‘warning’ causes issues with the add-on, so Bug 772319 was created. Simply put, this bug requests the patch/warning created as a result of Bug 277079 be removed.
Update October 2012: Bug 772319 has been resolved, this is will be fixed in Firefox 18. More info here.